So I'm wondering, after you get the Dual cast perk in the destruction tree does the damage bonus only work while using a single spell(IE both hands have Flame) or can you mix and match(IE One fire and One Ice)? the "combined" animation only plays with 1 spell in both hands but is that perhaps because they didn't add a combination animation for the effects or does it just not work together?

Dual casting is only applied when the same spell is used in both hands, so no mixing and getting the DC bonus. On the bright side (if you could call it that), the only real advantage of dual casting is impact, because otherwise dual casting causes your spells to only do a marginal amount more than casting the same spell in each hand individually, but increases the mana cost dramatically. So really, aside from wanting to stun an enemy every now and then, you're better off not dual casting anyway.