Flurry of Blows modifies your full-attack
It adds additional attacks, and applies a penalty. It is otherwise a normal full-attack like any other.
This means you may use any weapon you have available for any given attack. If you are holding two weapons, you can use one or the other for each attack you’d normally have.
Note that this is exactly the same as when you don’t use Flurry of Blows. If you have multiple attacks for whatever reason (BAB +6 or higher, the haste spell, whatever), you can use any weapon you have available for any given attack as you please.
On the flip side, you do not have any more attacks, normally, than you would with a single weapon. You simply get the attacks you would otherwise have. Those can be the ordinary attacks of your full-attack, or extra attacks from Flurry of Blows, or whatever. Simply having two weapons does not give you extra attacks.
To get extra attacks, you have to use the Two-Weapon Fighting combat option (and then you are subject to its penalties and restrictions). In particular, Two-Weapon Fighting is an exception to this “any weapon for any attack” bit. Note that this is more than just having two weapons; you have to actually choose to use Two-Weapon Fighting, or none of the following applies. When you do choose to use Two-Weapon Fighting, it changes your full-attack (much as Flurry of Blows does): it grants an extra attack and applies a penalty, and it also applies a special restriction on which weapon you can use for which attack: specifically, your bonus attack from Two-Weapon Fighting has to be a different weapon than the one you are otherwise using.
Furthermore, under Pathfinder rules, Flurry of Blows is Two-Weapon Fighting (except it doesn’t actually require you to use two weapons). As such, the idea is you cannot use it and Two-Weapon Fighting at the same time. If this is the case, there is absolutely zero benefit to having two weapons, except I guess the ability to pick which one you want. You’re better off with a more-powerful single weapon than splitting your wealth between two.
Unfortunately, the rules here are notoriously vague and have required numerous amendments, explanations, Paizo contradicting themselves several times, and finally overruling previous rules entirely because they never made sense. This is actually one of the most famous fiascoes in Pathfinder history, and the rules are left a mess as a result. I suggest just returning to the 3.5 rules, before all this started: Flurry of Blows and Two-Weapon Fighting are completely separate things, combine them if you want (taking even more penalties and gaining even more attacks). This is still a bad idea, since it is a very weak option, but at least the rules are clear, make sense, and there’s actually, theoretically, a benefit to dual-wielding as a monk.
If you're looking at a single melee attack, you're absolutely going to need at least 17 levels in Rogue (Assassin) for Death Strike. I can't think of anything in the game better for increasing single-hit damage than the deadly combo of Sneak Attack (9-10 d6), Assassinate (automatic crit), and Death Strike (double damage). I'd take two levels in Paladin on top of that for smites, and then your final level could be Barbarian (for rage damage). Additionally, take the charger and martial adept feats.
Average Damage without items: 222
Max Damage without items: 364
Damage Breakdown
- Rapier: 1d8 + 5
- Sneak Attack: 9d6
- Smite: 2d8
- Rage: +2
- Charger feat: +5
- Maneuver (several choices): 1d6
- Dueling fighting style: +2
This gives us a total of 3d8 + 10d6 + 14 damage. However, if we assume ideal conditions this Rogue will be attacking a surprised enemy that hasn't taken a turn yet, so the attack will auto-crit. That brings the damage to 6d8 + 20d6 + 14. That averages 27 + 70 + 14 = 111 damage (182 with max rolls).
Death Strike doubles that damage, for 222 average or 364 max.
Now let's add in weapons, items, and enchantments that could boost it above those numbers:
- Vorpal Sword would add +3, as well as +6d8 on a nat 20
- Enlarge: +1d4 damage (cast by a friend, or potion)
- A library of Manuals of Gainful Exercise would add another +5 from STR
These would add a total of 1d4 + 6d8 + 8 to the numbers above. Auto-crit to 2d4 + 12d8 + 8, and double, and you get an additional 134 (average) or 224 (max)
Then add Purple worm poison for another 12d6 damage (that doesn't crit or double, thanks Jonatan) - 42 average or 72 max.
This gives us a total bonus from weapons & spells of 176 average or 296 max.
Average damage with items & spells: 398
Max damage with items & spells: 660
Best Answer
You can accomplish seven attacks in a turn while still raging.
You mention you wanted to keep the theme of a barbarian, so I took that as you had to be able to employ the attacks during a Rage if available (i.e. no spells or concentration). With that restriction, your intuition to use monk is a solid one. The best build is as follows:
The Character
Carlos is a level 20 character with the following classes:
The Attacks
This build is intended to provide the most attacks throughout the career of the adventurer.
Now, Carlos can use any weapon and still get unarmed strikes from Flurry of Blows, but they will not get the extra attack dice unless he is wielding a monk weapon. Picking up the Tavern Brawler feat (perhaps as a Human (Variant)) mitigates this somewhat, but doesn't help when the attack dice starts to get larger as Carlos progresses as a monk.
Initially, Carlos acquires his barbarian identity with the best Primal Path for attacks:
Levels 3-7
At these levels, Carlos can benefit from the Ki feature to use Flurry of Blows allowing for these attacks:
When Carlos no longer has ki points, he can use Frenzy to give access to these attacks
If Carlos lacks rages and ki, he can switch to a monk weapon to benefit from Martial Arts for these attacks:
Levels 8-19
These levels are mostly dedicated to helping Carlos acquire more ki points, so he can use Flurry of Blows more often. In addition, he picks up Extra Attack at level 8, which while it is a little delayed, does give him an extra attack. Here's his attacks while he has ki points:
When out of ki points in Frenzy:
When out of rages and ki points:
Level 20
Finally, you reach this characters capstone potential where the Intoxicated Frenzy allows for seven attacks:
This is only possible when ki points are available, but with 7 ki points between rests, Carlos can use Flurry of Blows quite often.
Magic Items
Few magic items help a lot with extra attacks. However, Carlos may be interested in picking up a scimitar of speed which will allow for a more powerful attack than the Martial Arts option when out of ki points and frenzy:
A scimitar, however, is not a monk weapon meaning you will not benefit from Martial Arts's benefit to unarmed strikes when using it in other attack-sequences (such as when using Flurry of Blows).