Well, for dual box, I'd like to use mage/mage combo, SPriest+warlock combo. 2xmage's damage is great, as for Shadow priest and warlock, the dots will make trash die much faster.
But why not use triple box? it's awesome for prist+mage+warlock combo, you can have free water/food, portal, resu, tank(pet), and various buffs.
Obviously you need another new account