Well, there you have it. Your second weapon could act as a shield. Your primary weapon speed would increase, because you dont have a big ol' shield in the way, but your blocking ability would still be limited to melee only. So you could have a 2 hander, with normal speed and melee blocking, 2 one handed swords, with faster attacking and melee blocking, or 1 one handed sword with normal speed and full-range blocking.
That's pretty balanced, I think...
The 2 one handers would do less damage than the 2 handed sword, but you would attack faster. To be able to block missiles, you would need to give up some speed and damage, though.