Generally speaking, yes, he could
Two-handed weapons require two hands, shields require one. If you have at least three, that means you can use both.
Shields do not have penalties for being in the “offhand”
That’s where they usually are, regardless of what weapon is otherwise wielded. Also, note that “offhand” doesn’t actually mean anything unless or until you choose to use Two-Weapon Fighting to gain an extra attack.
If you have more than two arms, you usually treat all hands except one as offhand. This again only matters for Multiweapon Fighting.
Tower Shields are really bad
Note that Tower Shields have lots of drawbacks: −2 to all attacks, huge Armor Check Penalty, tiny Max Dex, and they weigh a ton. They are rarely a good idea to use. I’d encourage your player to avoid it, not because it’s too powerful, but because it’s not powerful enough. A solid Heavy Shield gives half as much AC, but you ditch most of the problems you get with a Tower Shield. The Standard Action for Cover option is just not even remotely worth it.
And this is ignoring the fact that Aegides don’t get proficiency in Tower Shields, so he’ll have to multiclass or burn a feat to get it. That’s a big deal; feats should get you more than the option of using equipment that isn’t better than what you already could use.
The Aegis Customizations actually give two arms
As specified under the lesser extra arms customization, these options give a pair of arms, not just one. Note, however, that shields do not stack with one another, so that’s of limited benefit. Lesser extra arms can carry things, but not use them, so that is insufficient to actually benefit from any shield or weapon. Actually using a shield requires extra arms, minimum Aegis 5 and requiring a total of 3 points (2 points for extra arms and 1 point for lesser extra arms which is a prereq). That’s a pretty big chunk of a 5th-level Aegis’s available customization points.
Note that the fourth arm is difficult to use here. He could have a one-handed or light weapon in it, but unless he’s using Two-Weapon Fighting (and therefore has way more Dex than you’d want on someone who has a Max Dex of 2 on his shield, or is taking ludicrous penalties for TWF without the feat) he isn’t able to use that alongside his earthbreaker, so there wouldn’t be much point. Even then, it’s going to take another −2 penalty on attacks just for being in the extra arms; it takes greater extra arms (minimum 8th level, and 6 customization points) to avoid those penalties.
(If it comes up, TWF penalties are based on whether or not he has a Light weapon in the offhand; that his main weapon is two-handed does not affect these. Note he could do this without extra arms if he used Armor Spikes as his Light weapon. Thus, the probably-optimal way to use greater extra arms is Two-Weapon Fighting with a Two-Handed weapon, a Light weapon, and a shield, to minimize TWF penalties. But other than the shield, you could just use Armor Spikes to achieve the same thing, so this is not especially impressive.)
None of this is particularly overpowered
You have a 5th-level character with, as a sizeable portion of his accumulated class features, the ability to use a 2d6-damage weapon along with a shield. Note that a Rogue at this level could easily have 3d6 Sneak Attack damage, on top of a base weapon damage that could be 1d6 or more; sure, some things are immune to that (though quite a lot fewer in Pathfinder than in 3.5), and he has to do a little work to make sure it triggers (though Flanking is usually easy and to everyone’s benefit), but it’s still literally twice as much damage.
He does have more AC, plus the DR of the Juggernaut Armor, but those are his class features, and are a big part of the reason why he’s doing half as much damage, has half as many base skill points for a much smaller list, and so on. And if he actually goes for the Tower Shield, his huge penalties to attack and many skill checks are even bigger problems. Note that for the same penalty to attack, your typical Rogue could double his damage by using Two-Weapon Fighting, so now his damage is quadruple what the Aegis gets.
The Aegis gets to be really, really difficult to hurt with physical attacks. That’s his schtick, that’s what the class is good at. It’s a pretty narrow schtick and I worry that he’ll find that smart enemies just ignore him since he’s a pain to kill but has low damage output. Maybe he’ll invest other customizations towards improving that, but Aegides aren’t exactly amazing in that department; he’s never actually going to catch up with the Rogue who gets his dual-wielding full-sneak-attack. That’s the Rogue’s thing; this is the Aegis’s. I recommend against punishing players for being good at what they’re supposed to be good at.
Taking a level or two of fighter is a mark of desperation for feats.
Taking more just doesn’t make sense; a feat per level is weak, but if you desperately need feats it can be the right choice. A feat every other level is worthless.
I’m going to answer based on pure fighter, but only after the caveat that I’m taking this as an arbitrary restriction that I would never recommend holding to.
The Defender Role
This is a concept from 4e, where fighters and other Defender classes could meaningfully protect allies and punish enemies for ignoring them.
3.5 does not have any such concept. If you are big, hardy, impossible to hit, with tons of HP, enemies will ignore you and go straight for the real threats. You have almost no way of stopping them from doing it; only magic does that at all well. Tripping is the best bet you have, but it’s not great.
Alternate Class Features, Variant Classes, Substitution Levels
Alternate class features are the only hope here; otherwise, there really is nothing that the fighter has to offer. I suppose you’d get Weapon Supremacy just to say you had.
Dungeoncrasher
This ACF from Dungeonscape replaces your 2nd-level and 6th-level bonus feats with the ability to deal massive damage by slamming people into walls with Bull Rush.
Zhentarim Soldier
These substitution levels from the Champions of Valor web supplement gives you Skill Focus (intimidate) at Fighter 3, extends the duration of the demoralize effect of Intimidate at Fighter 5, and allows you to demoralize as a swift action at Fighter 9. This costs nothing (excepting that you have to keep taking fighter levels, which as discussed is actually a very high cost), and the 9th-level one is actually good.
Between these two, you can almost justify 9 levels of fighter, since the swift-action demoralize is a fairly solid ability. Or, would be if everything and its brother didn’t ignore Fear effects...
Tactics
Fighters have access to three good tricks, more or less. As a pure fighter, you might as well attempt to do all of them, and maybe toss in some other ideas.
Charging
Barbarians do it better, but it is what it is. Wants Power Attack, Shocktrooper (Complete Warrior), Leap Attack (Complete Adventurer), and Battle Jump (Unapproachable East). Really, really wants at least one level of barbarian and warblade, but whatever. Lance is the weapon of choice if you go mounted, otherwise any two-handed weapon is best. Using a lance kind of pigeonholes you into not tripping, so just going with the trip weapons is probably better.
Dungeoncrashing
Only available with the ACF, this mostly relies on Bull Rush, so Power Attack and Improved Bull Rush are necessary. Shocktrooper (Complete Warrior) is crucial for the directional Bull Rushes (you can angle people as you push them), while Knockback (Races of Stone) allows you to Bull Rush on every attack rather than as a standard action. Weapon doesn’t matter for Bull Rush.
Intimidating
Only viable with the Zhentarim Soldier levels, and even then, a disturbingly large selection of foes will outright ignore it. Wants Imperious Command (Drow of the Underdark), as well as the Never Outnumbered skill trick (Complete Scoundrel). Max ranks in Intimidate, don’t completely dump Charisma.
With the Fighter 9 replacement from Zhentarim Soldier, lets you spend a swift action on the first round to attempt to demoralize everyone you threaten, and if successful leaves them cowering for 1 round. Cowering opponents are extremely vulnerable, so this is a really good trick when it works. After the first round, you can continue to use swift actions to demoralize, but only against single targets.
Unfortunately, Imperious Command requires Cha 15. Unless you roll for stats, and roll absurdly well, that’s just not going to happen until very late in the game, because Cha is your least, or at best second-least, important ability score. The only thing it improves is your Intimidate check, which is mostly about your ranks and bonuses from other sources, not your Cha.
Also, Tome of Battle has a new mechanic for Intimidate, called Duel of Wills. It’s a thing you can just choose to do, for free, at the start of combat. Since you’re pumping Intimidate and most don’t, you get some free bonuses, though they are small and fleeting. Still, free.
Initiating
Martial Study and Martial Stance from Tome of Battle are fighter bonus feats. You’re limited to only three of the former, which sucks, but there are some great options.
Obviously, you would be massively better off just taking levels of crusader or warblade.
For maneuvers, shadow walk would be awesome, iron heart surge and white raven tactics are excellent, charging minotaur strike fits in well with dungeoncrashing, etc. Defensive rebuke is one of the very-few true aggro-drawing abilities in the game. There are more. For stances, thicket of blades makes it much harder to get around your attacks of opportunity, and that is really important, because your true purpose here is...
Tripping
The bread and butter of any lockdown build. Combat Expertise, Combat Reflexes, and Improved Trip are necessary; Knock-down and Stand Still are solid choices. You need a weapon with both reach and tripping ability: the guisarme or spiked chain are your best options.
This is how you really lock down a fight. Intimidate’s a nice trick when it works, but it often won’t, and charging and/or dungeoncrashing are usually much more about damage than they are controlling the battlefield.
Tripping means anyone who wants to get past you, has to do it very carefully. You can trip on your attacks of opportunity, which tends to prompt another attack of opportunity when they get up—the AoO happens before they actually get up, though, so trippping at that point has no effect and they still can get up afterwards.
Races
You want to be big and strong, but you don’t want to be stupid because you need that Int 13 for Combat Expertise. Two levels of barbarian would get you around that, and massively improve your life, but oh well. At least it ensures you’ll have skill points for Intimidate and Jump.
Also, when you’re taking levels in a class as weak as fighter, level adjustment may be worthwhile. It’s usually awful, but maybe LA +1 can get us something good...
Goliath
Oh yeah, it can. Goliaths are from Races of Stone, and have both LA +1 and the Powerful Build ability, treating them as a size category larger for a lot of purposes (mostly everything except space and reach, which is a shame but oh well). That means +4 on Bull Rush and Trip checks, and slightly higher damage. The +4 is the important bit.
They also get +4 Strength, −2 Dexterity, and +2 Constitution. The −2 Dexterity definitely does hurt, but it’s rare to make more than 2-3 attacks of opportunity in a round, so you can survive it. The +4 Strength and +2 Constitution are big helps, though the Constitution is misleading because you also have one fewer level to get HP from.
Mountain Movement, the ability to always count as running for jumps, is also useful to you. It will allow you to trigger Leap Attack much more easily. If you can get it, though you probably can’t without a warblade dip, sudden leap would also benefit.
Half-minotaur
From Dragon vol. 313, this LA +1 template makes you actually Large, which means you get space and reach, which are very, very good for improving your threatened area.
Ability scores are Strength +4, Constitution +2, and Intelligence −2. Again, the hit to Intelligence hurts, but probably even less than Dexterity: you just need to make 13. Even if you start with Int 14 and put your +1 there, you can probably work with it.
Since this is a template, it can be applied to any of your other good choices. Going LA +2 as a half-minotaur goliath is interesting, but probably a bad idea.
Dragonborn warforged
Dragonborn is an LA +0 template from Races of the Dragon, which is cool because, hey, no LA. It also replaces the majority of your racial features, with the exceptions of ability score changes, subtypes, and movement modes. The subtype thing is important.
The dragonborn applies +2 Constitution, −2 Dexterity. Again, the Dexterity hurts.
What do we get for it? A few random things, nothing too special, but we get to choose a Draconic Aspect. Heart and Mind are worthless (a weak breath weapon or minor bonuses to select Will saves), but the third option is Wings. This is the correct choice; a fighter tends to have really, really big problems getting into the air, and this straight-up solves that problem.
Meanwhile, the race you choose usually doesn’t matter that much, since you don’t get to keep much of it. The warforged race is an exception, because most of their racial features aren’t features of the race, but rather of their Living Construct subtype, which Dragonborn lets them keep. It’s cheesy as hell, but you’re a freaking single-classed fighter, it’s OK.
Warforged offer a smorgasbord of immunities (poison, sleep, paralysis, disease, nausea, sickened, fatigue, and exhaustion) that will make your life better. It also means you don’t have to eat, drink, or sleep, which is nice. And you get +2 Constitution, while losing on −2 Wisdom, −2 Charisma—two ability scores, but your two least important.
Combined, you’re looking at all those immunities, flight, −2 Dexterity, +4 Constitution, −2 Wisdom, −2 Charisma.
And you could throw half-minotaur on there for +4 Strength, −2 Dexterity, +6 Constitution, −2 Intelligence, −2 Wisdom, −2 Charisma, as well as Large size and some other minor benefits. This is probably your best choice.
Human
Just saying, it’s viable. Think you have more than enough feats? Guess again; you’re going to need everything you can get. True, though, things you can’t get from feats—like size and flight—are more useful.
Conclusion
Half-minotaur dragonborn warforged fighter 20, with the Dungeoncrasher ACF and Zhentarim Soldier substitution levels. The penalties to Dex and Int mean some awkward choices on ability scores; you may hold off on Combat Reflexes until later, when you can get some gloves of dexterity to make better use of it. Late game, even the 36,000 gp for the +6 version is quite affordable.
A 26 PB might be (base) 14 in each of Str, Dex, and Con, and then 15 in Int, while leaving 8 in Wis and Cha. Having −2 to Will saves (and Intimidate checks, but that matters less) is bad here, but you don’t have a lot of option. The solution I’m using will be Steadfast Determination from Player’s Handbook II, to switch Will to using Constitution. At 32 PB, I’d push Cha to 14, for something like this (after racial mods):
- Str 18
- Dex 12
- Con 20
- Int 13
- Wis 6
- Cha 12
Not exactly amazing for LA +1, but we have a lot of requirements. As it is, we’re going to have to stick 1 into Cha from levels, and get a amulet of charisma +2, before we can get Imperious Command.
Other ability score bonuses should go to Strength.
If you are literally stopping at 12th, it may be worthwhile to just dump either Dex or Cha and give up on either Combat Reflexes or Imperious Command. Dumping Cha is the better choice, though it makes less use of your few unique features.
Anyway, we are a Large Monstrous Humanoid, with lots of immunities, and from level 1 can glide. At level 6, that will become flight. The LA does not count towards that, sadly.
We get 3 skill points per level. Intimidate, Jump, and, I don’t know, Handle Animal can be our skills. Cross-class ranks in Balance, Tumble, or Use Magic Device might be better, maybe.
All of your tricks are weak at the beginning of the game; you only get one AoO, your Intimidate ranks are barely beating your penalty to Cha and you can’t get Imperious Command, and dungeoncrash doesn’t really shine until Fighter 6. Still, the Fighter 2 version of dungeoncrash is solid enough, so that’s probably where you start:
LA +1, Power Attack
Improved Bull Rush
Dungeoncrasher I
Skill Focus (Intimidate), Endurance—you are literally immune to everything the Endurance feat helps with. See if you can get the DM to waive it as a requirement for Steadfast Determination, it would help massively.
Combat Expertise
Extended Demoralize, Never Outnumbered (skill trick)
Dungeoncrasher II, Steadfast Determination
—
Improved Trip
Swift Demoralize, Imperious Command, absolute last level where fighter gets anything meaningful
Knockback
—
Shock Trooper, Combat Reflexes
—
Knock-down
Martial Study (foehammer)
Martial Stance (thicket of blades)
—
Leap Attack
—
Yes, you get nothing at 20th level. Sorry.
For equipment, I recommend:
+1 guisarme, 4,309 gp—it reaches, it trips, it’s everything you want without burning another feat on a spiked chain.
masterwork spiked gauntlet, 305 gp—just so you can attack inside your reach, if necessary.
+1 full plate, 3,650 gp—you have heavy armor proficiency and your Dex is meh. So you want this. Unfortunately, dragonborn flight doesn’t work in heavy armor. Hopefully between a literally-pointless Endurance feat and the tooth of Savnok (see below), your DM can see their way to ignoring that limitation, at least for you. If not, you have to go with a +1 chain shirt (a breastplate is garbage; lots of penalties for just +1 more armored AC—not worth it). Eventually, you will be able to afford +1 mithral full plate, solving the problem.
tooth of Savnok, 2,000 gp—eliminates the movement penalties from armor. You’re required to never take off any armor you wear, but with the Endurance feat, you never have to. Only relevant if you get wear medium or heavy armor, which really should only be full plate, which as noted you might not be able to use. (Tome of Magic)
amulet of charisma +2, 4,000 gp—to enable Imperious Command, sadly.
belt of battle, giant’s strength +2, 16,000 gp—extra actions. Also, bonuses to initiative and Strength. (Magic Item Compendium, both for belt of battle and the rules that allow it to also have +2 enhancement to Strength)
third eye clarity, 3,000 gp—prevent or remove a lot of nasty effects, including daze and stun. (Magic Item Compendium)
anklet of translocation, 1,400 gp—quick, cheap mobility. You need it. (Magic Item Compendium)
That’s about 35,000 gp. You have 36,000 gp for 9th-level WBL, so that works out pretty well. Other good options include a belt of healing or scout’s headband. The fearsome armor property from Drow of the Underdark is a bit wasted on you (you already have swift-action intimidate, so the move-action version isn’t that useful), but +5 to intimidate isn’t awful. Getting gloves of dexterity would be nice but you neither have the money nor the feats.
Also, be sure to keep abreast of the usual answers to the typical problems that your Living Construct subtype and/or wings don’t already cover.
Best Answer
A Dual Shield fighter is possible, but is restrictive and fiddly.
First, when making high-concept stuff like this, it's critical to be aware of potential pitfalls of your character. Take a look at this guide to get a sense of the traps involved.
To realize this, my first preference would be to refluff some more "standard" version of the fighter into a double-shield fighter. Holding two shields does nothing for your defensive capabilities. But, with that said, the feats of superior proficiency "spiked shield" and midnight blade student make dual strike quite... interesting. You can "shield bash" two people a turn. Of course, the problem is in good positioning and actually doing damage. While entertaining, the dual strike fighter tends to be severely "one-trick pony"-ized.
As a first time player, I would instead look towards the Knight-variant of fighter. There's less "fiddly" stuff, and the idea of a heavy shield + spiked shield can serve you quite well. It's also very very hard to go wrong with Dwarven Weapon Training, with a Heavy shield and "gauntlet axe" (if you want to fit your personal style) and just say the "gauntlet axe" is a razor-sharp shield.
Two shields as "important" only really comes into play with the Snapping Tetsudo Paragon Path, a relatively uninspiring choice, unfortunately. To be fair, Dragon Magazine 385 has a significant number of "shield-as-weapon" choices, but few of them require two shields. All that being said, I would still go with knight + dwarven weapon training, and either pretend a hammer or an axe was a shield, depending if you want to be bashy or slicy. The mechanics fit better, and the thematic imagery can be just as vivid.