Pally priest combo. Teach your wife how to tank and you go a disc/shadow hybrid. Prot pally is the training wheels of tanking and is a perfect gateway to bringing anyone into a new role.
Between AOE potential,multi dotting,healing and ease of dungeon filling theres very little downtime which is the biggest slow down.
I like Paladin+Warlock combo a lot for leveling. Priest+Warlock is good too. But I also think Priest+Paladin is good too for a tank/healer duo for dungeons. I dont think you can go wrong with any of those duos.
Any healer + any class that can tap mobs fast imo. Hunters, locks, shadowpriest all have an easy time with tagging mobs and hunters and locks also have pets who can help you tank mobs as well as being 2 of the most powerful classes come TBC. any tank + any healer is also a good option if you plan to use dungeons for leveling especially once you hit 60+.
I enjoyed playing Destro lock + Holy priest with my gf. Priest stacks spirit and specs 5/5 Spirit Tap. Just let her last hit with wand or something to keep ST rolling and you’ve got an infinite mana battery to power your lock.