Firstly, apparently the damage you see on your character sheet is the AVERAGE of both weapons - so you damage might have seemed to go down, but your actual damage is double what it shows...
But if you haven't put any points into dual wield yet, you're suffering a serious disadvantage on your off-hand weapon. Spend enough points and your off-hand will become a bonus instead of a penalty.
Finally, each +damage enchant you have (either magic or physical) will affect both weapons - this includes +damage enchants on the individual weapons. After you spend enough points in dual wield, you'll be more than doubling the effect of each +damage enchant that you have!
