Hmm, apply it to real world settings as I always like to...
I don't know if I could hold two automatic weapons in both hands and have any degree of accuracy with them... I could certainly try, but I think I would have enough trouble with one...
Let's say we take away the autos, and stick to single fires, only handguns are left. Again, I am not sure that I would accomplish much with two handguns in both hands, I think I'd be so focused on getting my dominant hand to perform somewhere near normal levels (requiring two hands) that my 'on' hand would not only suffer, my 'off' hand would be sure to miss (just because of my mental focus on my 'on' hand)...
Twin weapons, if it is instituted, should result in a drastic reduction in "to-hit" rolls, on either hand, I am talking in the -50% range... In fact I think it should (IF it is even implemented) be a perk. A perk with serious ramifications on your skills.
BAD idea, even with serious consequences... Bad or good idea, it still should have SERIOUS consequences...
--pate