When you duel wield, the average of the weapons damage is displayed in the inventory, so you would need to double it to get the damage per attack. A good two handed sword may do more damage than duel wielding, depending one what one handed weapons you have, but duel wielding is still an effective style early in the game. It gets even better once you can start putting enchantments on your weapons.
When I played a duel wielding warrior in D2:ED, early in the game I selected my off-hand weapon more for the bonuses it had than the base damage. Once I started to get the duel wield skill up and reduce the off-hand penalty, though, I gradually started putting more emphasis on straight damage.
