Hmm, good question. I would have thought that characters only get penalties for wielding to weapons at the same time, and in that case for
both weapons. That's how I would have done it.
On the other hand (ahaha), even if you equip your sword in the left hand, that doesn't make you a lefty yet... there are other animations in the game, and they will all be right-handed animations. For example, a character who picks a lock will most likely do so by equipping the lockpick in the right hand, and maybe a probe in the left hand or whatever. Or for two-handed weapons: You can't just equip that weapon in the other hand, it will always show the animation that is meant for right-handed characters. (Although I'm currently not sure which one that would be - left hand higher than the right one, or other way around?)
So... point is, even IF there is no penalty to holding a weapon in the left hand, the game still won't allow you to play a left-handed character UNLESS there's a specific option somewhere that mirrors all the animations.
during medieval times, fighting with yr left hand was a symbol of fighting for the devil.
Nice.