The sword held in the off hand will always do less damage if it has the same weapon. (My profile is currently out of date) I have 2 northern shortswords equipped, and the one in the off hand does less damage than the one in the main hand. However, the extra damage is always nice, and the high number of hits is great for fighting casters. In the end, I find dual wield better for soloing, and 1 sword + shield better for large parties so I make a better tank.