[Waedawen]

Enchanter + Druid (my favorite)
Enchanter + Cleric
Necromancer + Enchanter
Necromancer + Magician
Necromancer + Druid
Enchanter + Rogue
Necromancer + Rogue
Druid + Rogue
Wizard + Druid

First 4 combos can kill absolutely non stop inside a dungeon with the right spell sets, last 4 can kill extremely effectively, perhaps nonstop outside of dungeons probably at level 10.

EDIT: I don't include clerics because I simply don't think clerics bring enough to the table when you're partnering up. They heal people, big deal. What they really open up is the ability to get the group expanded easily because you don't have to deal with getting some pre-Madonna of a cleric to come join your group.
The exception is with an Enchanter, where the power of complete heal is truly allowed to shine. But that will be all they contribute to the duo, essentially.