wow thats a really cool idea, 2 multiboxers in one 5man, hah!
i guess your class setup would depend on what you wanna do, pve/pvp... but all i can think of is ellay with his dedicated arena healer pewpewpewpewheal!!!
and while i haven't been back at 70 with a healer for a while... i'd maybe consider a druid over a priest... but i like hots, not sure what is technically better right now.