I don't think that plan would be overpowered. Therefore, I don't consider it abuse. I think having one mage play support while the other goes on offense is a valid strategy in a team game. If the priestess is really causing trouble, I don't think the Warlock could block your team from getting to her for too long.
Extreme scenario, she walls herself in with a Grimson Deadeye, Archer's Tower, and a Mage Wand with Heal attached. She may be able to get off one attack per round with Grimson, but otherwise her functionality is limited purely to healing. Meanwhile your team would be able to simply out produce them I expect. You could get out more enchantments, conjurations, creatures, etc than the Warlock could produce, and he wouldn't be able to deal with everything you pump out on his own. You'd eventually overwhelm them.
With that said, even though I don't think it would be a problem, it is something that should be playtested. I don't expect I'll have another game until after the holidays myself, but I will see if my group is willing to try out some kind of Warlock/Priestess strategy when next we do battle.