So when dual wielding I think having a war axe in the off hand sounds like the best option since it doesn't need to be quick and the bleeding effect might add a few damage over time.

Is there any reason to use a dagger in the off-hand? Any other combination ideas?

Should be whatever you're perk'd for unless you're taking all the various weapon perk types. Technically as it's based on 'on hit' damage the highest damage dual wield weapons will likely be two maces. I don't know how or if the weapon speed will effect the double swing power attack animation though.