How does this work exactly? Do you hit twice as often, one time with the left and one time with the right? Or do you add up the damage from both weapons in each hit? Or what? I can't seem to figure it out.

You get to make 2 attacks (or more)! Depending on weather you're proficiant, specialized etc in duel wielding you'll suffer penalties to you're THACO though. More with the left hand. I dont know all the stats by hart (someone else help out please) but if you have grand mastery in duel wielding you suffer no penalties with the right hand and -2 with the left. I am not sure but I think if you would make more attacks per round, that is in reference to your right hand and that you can only make one attack per roun with your left. Someone please correct me if I'm wrong, I'd like to know too

Gives one Xtra attack pre round