Well, melee classes are pretty unforgiving in the slave positions. Hitting something breaks follow, so you'd have to account for and maneuver the mob to them. Casters just have to face the target, so it's much easier. ele sham + ret pally or rogue is much better, and you still can be dropping windfury.

It might even be better to level 2 of the same class/spec for a while till you get used to the new challenges faced while playing more than one toon. And don't be scared of leveling classes you already have, that knowledge would just help you in the long run. If your priest is a healer, Spriest/warlock could be the funnest spec for you to play, even comes with it's own tank - the warlock's pet.
I think the challenge of micro-managing two characters at once would add a new level of excitement and difficulty to the game that I feel it badly needs.
You'll be pleasantly surprised here. It's much more fun/challenging then leveling solo. Perfecting macros and keybinds alone is challenging, something you never really have to worry about solo. Just be careful, it's a slippery slope from 2 to 3 to full-on 5 boxing.