Try two mages - not only do you have a ton of utility, but you can dual spec both, and make both arcane, and one fire, and one frost, so you have all the bases covered, like running into arcane immune mobs.

Rogue and Druid would be fun, both can stealth, and the Rogue would benefit from the Moonkin buff.

Two Pallys is dead easy - one heals, one melees, or for tough fights, both melee. They're OP right now, take advantage.

Two Locks could be fun, but you'd have no healing - two drain locks could go forever, and you could run a demon and affliction combo - the pet tanks, bot dot the mobs up.

Two shaman I've done, Ieveled two together to around 25, very easy, very fun, lots of utility, and with dual specs, you have everything covered.

With RAF, leveling is so easy, roll one of each, and try all the combos out.

Then, once you get 55's on both accounts, try two DKs - lots of fun there!