For just two characters, I don't see any point in going with a tank + healer. I would only do that if I would box a whole group.

I would highly recommend two mages, both of them identically specced Warlock / Necro for 99% of all play. In Necro you want Soul Purge and the upgraded pets. For a few things you may want Elemental and Chloro roles for better tanking and healing (but much worse dps).