I don't know why Flurry does not crit every hit in your tests. I tested it with dual daggers and backstab and it crits every hit.
Flurry: 26 + 24 + 24 = 74 damage
Basic attack: 49 + 29 = 78 damage
Apparently it's actually weaker than one basic attack while costing more AP.
If your character does not crit every hit with Flurry, maybe that's just how it works when you're not backstabbing.
From what I've seen, the base damage of two-handed weapons tend to be 1.5 or 1.6 times that of the one-handed variations of the same level. But when you're dual wielding, your offhand suffers -50% damage. So by a rough calculation, this means you should do about the same damage in one basic attack (2 AP) when you're NOT critical hitting. If you critical hit and have points in Two-Handed style, the two-handed weapon should yield more damage due to bonus critical damage.
