I've fenced against two-weapon styles a few times (dual dagger, dagger and rapier, cloak and rapier), so I can vouch for it usually being about one for defense and the other for offense, but...
Using two weapons, like using one weapon, is a game of cunning and tactics. Unlike a shield, having two weapons means if you defend to the right you're free to attack to the left and vice versa. It's sort of like hitting high then low, but you can hold your opponent's weapon at bay.
Of course, this tactical element of combat is lost in every rpg I've ever played, because it's abstracted into the die rolls. If your mechanics work in a way similar to most traditional rpgs, I'd suggest that beginner two-handed fighting gets a defense bonus, medium skill two-handed fighting a to-hit bonus (parry to create and opening and then strike) and only experienced fighters would get an extra attack.
I'd also note that off-hand penalties aren't as problematic as you might think, that a little practice minimizes the difference. I'm far from ambidextrous, but opponents have trouble figuring out which hand I prefer when fencing.