I think, you should not calculate the damages for 1 attack but for a delay of time.
(I don't have access to the demo, so everything I will say is based on what saw on videos or read on the forum. Correct me if anything is wrong.)
In a case when the accuracy is equal to the deflection, only the roll matter:
1-15-> Damage*0
16-50-> Damage*0.5
51-100-> Damage*1
Which give us an average of:
Domage*(0*15+0.5*35+1*50) = Damage*0.675
2H weapon do a damage between 14 and 20 so an average of 17. So with the roll chance it is 17*0.675 = 11.475 per attack
A fast weapon do a damage between 8 and 12 so an average of 10. So with the roll chance it is 10*0.675 = 6.75 per attack
(Here is the point where I am not certain)
I saw on a video where a character was dual wield two fast weapons (It has certainly "Two Weapon Style").
This character was able to attack 3 times while an attack of 2H weapon.
So for the time of 1 attack of 2H, we have 3 attacks of dual wield fast weapon.
So in term of damage it means:
2H weapon -> 1*11.475 = 11.475 (with x1.1 of Two Handed Style it is 12.6225 )
Dual wield two fast weapons -> 3*6.75 = 20.25
Even if you keep only 20% of the damage due to the DR, it is:
2H weapon -> 2.295 (with x1.1 of Two Handed Style it is 2.5245)
Dual wield two fast weapons -> 4.05
So apparently, unless I forgot something, dual wield two fast weapons makes more damage than a 2H weapon.
