Surprised someone didn't answer this for you. : ( Anyway, if it's going to be two casters running around, I'm guessing the conjy is going to be using the tank pet. (my knowledge of conjy's is very limited). If so then the Coercer is the best for your tank pet. Hate and dps buff alone is really nice. If you are going to run around with mage pet up for the Conjy, then the Illusionist would probably be better for you.
I 3 box using a conjuror, wiz, and and a Illy. This actually works pretty well. The wiz is great for DPS and the conj uses the tank pet, with some DPS support. The illy is great for utility and for crowd control if things get a bit out of hand. The buffs are nice as well. I used to 3 box with a conj, wiz, warlock but with the chanter replacing the warlock it is working out very well
