I saw a show on a martial art form (don't recall which one) some time ago where they went into a bit of detail on various weapons. Dual wielded weapons tend to be shorter, so someone using a single sword can have a better reach. A single sword has to defend against two weapons, but it has greater flexibility, can be switched (depending on the sword) between one handed and 2 handed stances, and having an arm free gives you better balance.
To be included in the game, a realistic / balanced dual weapon system would restrict weapon sizes (obviously only one handed weapons, but possibly excluding the largest such swords), require a high level of training (perhaps with sever defensive and accuracy penalties at first) and higher stat requirements and have a slightly slower attack speed than a single weapon (perhaps comparable with sword and shield). There are various things that could be tweaked so it would not be overpowered. I'd expect effective dual wielding to require lighter armour to allow freer movement.
