It's a waste. Dueling gives you only the bonuses if you wield only one weapon in one hand, and the two-weapon fighting gives you reduction when you wield two seperate weapons. Generally, if you take two-weapon fighting, you won't need to switch back to one weapon ever again.
Some might consider it a waste since you don't get the defense bonus unless you've got one hand free. But if you're so inclined, you can switch back and forth between two weapons/double-bladed weapon and a single weapon. Damage vs. Defense.
