If multi-attack allows multiple surfaces on a weapon to contact an enemy, you could kludge "dual-wielding" by adding large, 2-handed "pairs of blades" which are modeled as a single-weapon but fluffed as two weapons.
You wouldn't be able to multiattack with them. Think of it like scratching and punching, if you punch with your right hand in adventurer mode and select multi attack, then when you next go to attack, the scratch with your right hand option will be gone. Same for weapons, and there's no token to indicate that different parts of the same item are seperate so it imagines it like you're punching and you can't do two different attacks with the same thing at once.
However the [ATTACK_FLAG_INDEPENDENT_MULTIATTACK] might actually work when wielding two weapons, because then it's like the heads of a hydra. You can already multiattack with multiple weapons, just like you can multiattack with multiple hydra heads, so then adding independant multiattack would remove/reduce(?) the penalty. My main concern with dual wielding was that in the past dwarves would basically use their second weapon like a shield. If you gave them a copper axe and a masterwork steel axe and they picked up the copper one first then they'd attack with the copper axe far more often than the steel axe because the dwarven mind doesn't understand the concept of dual wielding and doesn't register the second weapon as a weapon. However, with hydras they almost always multiattack with multiple heads because they can with no penalty, so with that new token it might allow dwarves to actually use their second weapon regularly as they'd almost always multiattack. I don't think any testing has been done though it'd be pretty easy in fact I'm going to go do it right now.
EDIT:I tested it, and all I can really say is it's weird. I put that token on axes, and they were using both axes regularly and there might have been some multiattacks but I couldn't tell because most of the attacks were being blocked. However I also tried giving them two swords and they also seemed to use both interchangably and both times when I controlled a dwarf myself and tried a multiattack it seemed to be about the same effectiveness. I don't know.