If i had to lvl 2 chars i would probably try a warrior/DK/ret pally/ enhancement shaman or Feral + healer. since the melee can keep doing damage, preferably a fury warrior / DK, since they don't need mana breaks. I think a resto shaman would add most as the healer.

If you prefer casters i would not take shamans either, burst damage with just 2 isn't that great. I would try a hybrid + dps. Lock + Shadow Priest sounds like a great option, since the pet can tank, Vampiric Embrace from the shadow priest heals. Because of the heals the pet should be ok and if needed the Lock can heal the pet just fine since he/she gets healed by the Spriest aswell.

2 mages can be fun for AoE, good CC(sheep), but are indeed squishy and require manabreaks.
2 hunters, pets are good tanks and with one BM and one MM you can surely have some fun. Same goes for BMx2, big burst damage.

I think it's most about what you think is fun to play, if you want to have fun. If you only box 2 characters i think you won't gain a lot in pvp, since the disadvantages are quite big. Having just one slave following you you don't gain that much from it.