If I were going to dual box, I'd use one of these combos

Warlock-priest -- Dots, synergy, easy to control and similar in many ways, mindblast/Shadbolt+Shadowburn/SWDeath is deadly.
Frost mages -- because of shatters
AP/POM/Scorch build mage x2 -- lots of burst, lots of synergy
AP/POM/Scorch mage- Destro fire lock -- Lots of synergy, WotLK these two will synergize better than spriest/afflic, lots of burst.
Warrior-RestoShaman- healbot+windfury+warrior, yes plz.
Warrior-restodruid- Warrior + easiest healbot to dualbox.
Retpally-RestoShaman- Same as warrior +restoshaman, but less consistent dps, though more burst and survivability.

Can't think of any more off the top of my head right now.