What better than more of the same! But seriously, for dual boxing, I'd want at least one of them capable of rezzing, and healing, even if not specced for it. Paladin would be hard to box as anything other than a healbot, but they're durable, good single target healers, etc. Priest/Druid/Shaman can be more useful, any spec will work fine, just macro in some dps squences, and heal when necessary.

I'm at least gonna create 6 Dks for the fun of it though I will most likely only be leveling one. It'd be nice to have another tank, already have a 70 pally tank, and a 52 druid i was gonna spec feral. I have 4 healers, and 3 teams of DPS (mages shaman warlocks) and a few other randoms.

When my DK gets to outland, I will at least have him followed by 5 shaman, and I'll just do the quests twice (the kill ones are easy, and the named ones respawn so fast, I usually had to kill them 3-4 times anyway) I'll probably end up subbing out a shaman at a time for instances, run the place 5x, so each one sits out one run. The DK will get more exp that way, but that's fine. I have that 52 druid I can use to catch the shaman back up