In my limited experience in dual boxing, I'd stick with either a melee / ranged combo or ranged / ranged.

Warrior / shammy
ret Pally / Mage
etc...

War shammy combo will be nice when shammy get WF totem.

Right now I'm using a ret pally and balance druid combo, and I really like it. Eventually The druid will be a dreamstate healer with moonkin form and the pally will be prot.