well you main arm would only suffer a minor penalty... the off arm would suffer a bigger penalty. Then an additional penalty for mental management of keeping two pistols aimed at the same general target - if the engine allowed (which i think is an impossibility at this moment) to fire at two different targets, then the penalty is even higher.
You might have a bigger chance of hitting with the law of averages playing here (sorry no proof as I can't be arsed to do the math), but you'd waste twice the ammo.
Also I would reduce the PCs AC by some amount when using two pistols as the concentration requirements are higher focusing on useing two weapons instead of using you natural dexterity to dodge incoming attacks.
This is where adding an ambidexterity perk (or some "Desperado" perk) would be useful to combat two-handed penalties. Even better would be a perk that reduces the two-handed penalty, but perhaps detroys one's ability to use big guns or any single gun requiring two hands (dunno if that should be a perk, tho, as it offers a disad... it's more like a trait, isn't it in this last case?).