There is a way to use dual wield effectively and to deadly effect. However, I think it only works if you have the "Shadow Warrior" perk and are using dual daggers.
What I do is when I fight, I let an enemy come really close to me. Maybe let him hit me just a bit to make sure the gap between us is close enough. Right after that I crouch (Sneak) which will activate the "Shadow Warrior" perk (makes you become invisible for a really short time). And right after I turn invisible, I use dual wield power attack (hold left+right). If you do it right, it can finish a boss or a dragon in almost one hit.
The process is similar to sneaking behind an enemy, but this one is you suddenly become undetected right in front of an enemy. Use that short moment before they detect you again (around 2 seconds) to create a sneaky critical hit.
However, it is really difficult because you have to be really close to your enemy. And it is even more difficult with dragons, since they can detect you quite easily. But try it out, I killed several dragons with this trick already.
If you want to practice this, try practicing it with just a normal attack first. If you succeed in doing it against a normal enemy, sometimes it will turn into a finishing move cutscene (the one when you slice your enemy's throat from sneaking attack). By the way, this looks really cool. It is almost like you turn into a smoke, teleport behind them, and slice them.