I am dual boxing a shadow priest and a warlock. I think this is better for questing/leveling.

When you think about the fact that it's easy enough for any 1 class to solo content for questing/leveling, what the lock/sp combo gives you is non-stop, no downtime grinding.

I don't use the priest to do much offense outside of vampiric touch, vampiric embrace and shadow word: pain. What most of his mana is used for is replenishing the lock's lifetaps and the pet's health.

I can run through multiple mobs at the same time extremely easy and by the end of the fight be at full mana and health.