The "correct" answer is that your assumptions are invalid. In my experience, a defender needing to enforce her mark more than once per round suggests both an over-zealous defender and an under-protected party.

However, in practical terms, if you're even asking this question it means that the "correct" answer isn't particularly useful. A defender should, most of the time, try to occupy the "tender mercies" of two enemies. They can, given their defenses, survive two enemies attacking them. If your GM is undaunted by mark-punishment there are a number of options, taken from my memory of the art of defending. There are 5 excellent threads here.

To rephrase your question in the realm of the practical then, the question is not simply "how do I enforce marks more than once per round" but "how do I attract and maintain the attention of two monsters?"

This raises the question of "aggro" and "stickiness". Aggro is a measure of how much a monster is inclined to hurt you. Having a painful punishment is an excellent way of raising aggro. Having high defenses is an excellent way of lowering aggro, especially if you consider the relative defenses of the defender and the party. If the defender is a low-damage wall, most sane monsters will simply ignore the wall. Don't be a wall. It's boring. (I speak from bitter experience here).

Stickiness is, effectively, soft control which prevents monsters who would otherwise move away from ... moving away. The fighter has failed to draw enough aggro and now must remind the monster to respect her. Defenders... vary... in terms of effective stickiness. Forced movement combined with a terrain feature of a wall or difficult terrain is an excellent sticky-amplifier, as is causing creatures to fall prone.

Therefore, some notes on how to manage aggro and stickiness as each type of defender at each tier.

Battlemind

The battlemind functionally has two "tiers" which are "before lightning rush" and "after lightning rush."

Before lightning rush

From levels 1-6, the battlemind is a fairly normal defender. They have absolutely no "sticky" capabilities, allowing marked monsters the free run of the place. To counterbalance, they are also quite happy to follow monsters if they are foolish enough to shift. With the right power choices (if your GM likes provoking) they can also perform quite punishing OAs.

To engage two monsters, both intent on attacking more squishy members of your party, you'll want to rely on conductive defense and vicious cobra strike. Conductive defense is a punishment stacker, dealing out primary damage as punishment whenever the enemy hits an ally. If the GM really is suicidal, combine it with mark of storm and flail expertise. Then they're prone and next to you. In order to attack someone else, they will have to provoke an OA from you (assuming no allies are in melee with them.) CD allows you to "sticky" one enemy by knocking it prone next to you with pseudo-punishment and keep another enemy marked. Don't forget the augment 1 to really discourage shifting. If you focus on this, your second at-will should be twisted eye for its augment 1: allowing you to use it on OA. (Or be a half-elf with eldritch strike). If enemies routinely violate your mark, go with a two-handed weapon. Not only will this lower your AC, (making you a more attractive target) but the BFWeapon in your hands will make most sane creatures think twice about giving you a free swing with it that debuffs them further.

After level 7 and lightning rush, the ability as an interrupt to whack an enemy who's silly enough to attack an ally within 5 squares of you is quite sufficient. The kicker here is to have a lightning (flail) with flail expertise and mark of storm (or be a half-elf). Then you can (if it's a melee attack) slide the enemy away from the ally, negating the attack, or, if they're a ranged attacker, get an OA then your lightning rush in. This should generate quite sufficient aggro, not to mention the proning and sliding shenanigans.

Cavalier

This class presents a few problems, to be honest. The defender aura is excellent for multiple-enforcement, and absolutely pathetic for actually doing your job of getting 2 enemies to attack you. It does, however, auto-damage whenever an enemy shifts out of the aura or attacks someone else while within it, so it does its job. The damage, however, is not sufficiently significant to worry an enemy who is attracted by the squishies surrounding you.

Righteous shield is a good 1/enc "no really, pay attention" but can't carry sufficient weight on its own. Given that your enforcement is OA based instead of II, shifting after your enemies isn't enough to maintain stickiness. The reduced daily count also isn't particularly impressive for this task. If you find yourself being in a position where the rest of your party is too squishy and the DM isn't respecting your radiance, your only real bet is to be human and grab power of earth and bolstering strike. Or, going by the recommendations of the charop guide, grab virtuous strike for power of arcana (white lotus hinderance is actually exceptionally good for stickiness if you can get your opponent against a wall) or power of the sun, which, to be fair, both does more damage and makes your radiance hurt far more, increasing your aggro, rather than your sticky. In higher tiers, it's about the same problem, but your dailies can compensate fairly well.

Knight

See above. Recently a series of feats for knights enhancing their power strike power have come out. Given that a knight performs an MBA on aura violation and slows on hit (you have hold the line, right?) there's no real "problem" with enforcing your aura against as many enemies as you want to hug. If you find that despite your quite impressive attacking capabilities they still want to be friends with your allies, you can invest in a lightning flail, flail expertise, and mark of storm to prone them, both when you attack with your MBA and when you punish them. Beyond that, the flail strike which prones them causes them to provoke when they stand from prone is absolutely delicious. This class is perfectly suited to multi-"marking" and insuring that enemies stick around. The only caveat is to consider your AC carefully, as you need to strike a balance between not being dead and having enemies dismiss your MBA as "weak" (more fool they). There's no real tier change in tactics.

Fighter

The fighter faces some difficulties in multi-marking, as noted in your question. However, fighters are past masters of both sticky and aggro-generation techniques.

In order to adequately assess a fighter, we must look at them in low heroic, high heroic, and paragon.

In low heroic, the best resource is the "battle-fury stance" By upping damage and dropping AC, you make your mark punishment non-trivially painful and increase the enemy's desire to attack you. This, combined with deft hurler cleave or dual strike is more than sufficient to lock down two targets with sufficient threat generation. If one target decides to provoke your MBA is more than sufficiently painful to punish him for that decision. Another option is the spear benefit of weapon master's strike, which causes them to provoke while shifting. This excellent power means that you can, quite literally, stop shifts (due to combat superiority).

If both enemies decide to break away (repeatedly) your best bet is to use tide of iron to position them against a surface. This way, they cannot shift-charge without provoking. While you can't tide both of them, mark of storm and a pair of lightning weapons (yes, this is starting to cost) with dual strike is more than enough to see the positioning through.

In high heroic, the sliding can be improved with rushing cleats a bludgeon expertise. With a slide 3 (from mark of storm or tide of iron), you'll be able to position your enemies exactly where you want them. Come and Get it (level 7 encounter power) is also effective in this regard. You may also want to investigate "Binding Style" which flat-out immobilizes the second target of dual strike. In this vein, you may want to consider a "net multiclass" to slow targets of your net attacks. Since a net is an off-hand weapon, it is an absolutely fantastic control technique, especially when combined with a slide.

In paragon, the level 16 gladiator champion dispenses with trickery and simply makes you undeniably sticky: Enemies cannot shift away from you. Period. Kulkor Arms Master rewards you significantly for proning opponents (quite achievable in many ways) and is considered one of the cheesiest fighter PPs. Son of Mercy is a bit single-target for your requirements, but is a lovely complement to dual strike for both damange and control of the person you don't have cornered.

Paladin

Paladins get both divine challenge and divine sanction. While the amount of punishment isn't particularly impressive, they can challenge and sanction different people, and accomplish multiple punishment actions in a turn, no extra stuff (beyond an at-will that can sanction) necessary.

Swordmage

Swordmages are.. difficult defenders to play. Assuming you're in paragon and have double-marking, your main recourse here is to mark (of shielding) someone and then ignore them, focusing your attacks on a second enemy. While there are no penalties for that second enemy to shift-charge away, sliding and/or proning that second enemy is a good "second best"

Warden

Wardens are absolutely fantastic at sticky, having a number of daily forms that create difficult terrain around them, as well as powers that slow (and feats that up damage of powers that slow.) Multi-target sticky is more difficult, and mainly is a result of form of winter's herald and treacherous ice. At-will sticky is from slowing a target that isn't pinned against the wall. Wardens have excellent PP options for difficult terrain and forced movement, the level 16 forced movement and the daily 20 polymorph are exactly what's needed in this situation. You may want to invest in feats which up the penalty to attacks from your mark, as multi-marking is trivial. Don't forget to take sudden roots. Mark of warding is one of them, as is razorclaw mark (not the best choice of race, but... interesting from an RP point of view). With a -3 to attack rolls and the soft control of slowing, immobilizing, and/or forced movement, you should be able to keep two targets engaged without difficulty.

In terms of enhancement bonus, players should start finding +1 gear almost immediately, +2 gear around level 5, +3 gear around level 10, +4 gear around level 15, +5 gear around level 20, and +6 gear around level 25. Note that any armor that has a high enough enhancement bonus to be masterwork should always be the best kind of masterwork allowed for its enhancement bonus. Masterwork armor is a math fix for the way player defenses tend to lag compared to monster attacks as party level increases; it is not something special that players should have to work to get.

In terms of special abilities, weapons that give a conditional bonus to damage (against enemies of a specific type, against enemies larger than you, on your attacks with a specific keyword, etc) usually give an item bonus equal to their enhancement bonus; weapons with an unconditional damage bonus are very powerful.

Armor and neck items that directly give extra protection are unusual and rather powerful; most give a daily item power, a skill bonus equal to the item's enhancement bonus, or both. If they do give a protective boost, the most common is a +2 bonus to 1 or 2 defenses when a common condition is met (you shifted this turn, you're a druid in beast form, etc) or against certain kinds of attacks (close attacks, ranged attacks from 5+ squares away, etc).

Remember: damage bonuses should go up as the party gains levels, defense & attack bonuses should not (except for enhancement bonuses).

A good item gives one, maybe two of: a very small always-on bonus, a moderate conditional bonus to damage/attack/defense, a moderate always-on boost to initiative or 2 skills, a daily power equivalent to an encounter utility power or capable of making an encounter power closer to a daily power in strength, or an encounter power equivalent to an at-will utility power.

If you want an more powerful item and/or an item with lots of roleplay significance, take a look at artifacts (which unfortunately are spread around in a variety of books) for some examples and ideas.

If you have DDI, an easy way to estimate how good items should be is to look through the compendium and see what magical enchantments already exist for that item at that level.