I personally prefer Melle > Master, Caster > Slave; however, it is completely personal preference. If your only goal is to get 10 toons to 85 it doesn't really matter what you use to dual box. any two toons can be dual boxed questing, considering you pretty much two shot anything there is. and Mage/Lock as arcane/demo would be so super easy its ridiculous. Felguard tank lock and mage destroy mobs. Your main focus has to be on what spec you choose. BM hunter for instance can solo heal his pet while your other toon destroys mobs.

All that said the most fun I have had when only playing two characters is with a disc priest and a tank, any tank, spec into Holy fire and smite healing and you literally only have to bubble your tank and then heal through damage. The damage is not half bad either.

Hope this helps.