I've tried the alt-tabbing route for two dps characters (moonkin and warlock) and for normal quests found that things were pretty much dead before I had a chance to do anything on the second character.

In instances obviously the mobs will die more slowly (if you are close to the correct level). I still think this would be quite hard though, even with TC and cleave on 3+ mobs my warrior is prone to losing one of them to the priests healing agro. Also I'm not sure alt-tabbing would be able to give you the reaction time needed sometimes to get heals off.

For outside instance I'd say you'll mainly be killing things with your warrior and using your priest as a health top-up (spec doesn't really matter at low levels for healing).

I'm not saying it won't work for you (my comp is fairly minimum spec wise, so I get a bit of lag when alt-tabbing), just in my experience it wasn't efficient levelling two characters via alt-tabbing.