If you want faster leveling, I didn't find warrior/priest very efficient, to be honest. The reason is because the priest doesn't help you really kill mobs faster, it just limits your down time. I would say warrior/pally or shaman/warrior are better since mobs will die quicker and you get heals.

I would also say dual boxing is kind of meh overall. I would just go all the way to 5. You can always cancel the accounts you don't want after you mains are at level.