Hmm, from what I have heard and seen, there are 2 incredibly well running caster combinations:
1. shadow priest + affliction/demo warlock
2. fire mage + destruction warlock
Melee always seems awkward while dualboxing. 2 Melees are hard to handle because of positional skills and 1 melee plus 1 caster/healer results in aggro or positioning issues. Of course, those are challenges, no problems, and people who dualbox love challenges, right? :D