I would go with lock/lock or lock/priest.

I played 2x mages to lvl 20 or so but it was a bit of a struggle for me. I also played 2x locks to lvl 20 and it was a breeze compared to the mages. The pets give you added options. Fighting elites become a lot easier. With the 2 mages I had real problems taking down the elites in Ghostlands, with the warlocks it was cake. Maybe I just played the mages wrong but their abilities seem more movement and positioning based, ie blink, cone of cold, etc, which are not boxing friendly.

Warlocks on the other hand have pets, fears, dots (and most of your dots don't require you to face the target) which all help with killing stuff quickly and safely. You also have life tap and dark pact (if specced) to help with mana regen so almost no downtime.

Mages can solo AoE packs of mobs but the techniques are very movement based and I don't know if you would be able to pull it off boxing multiple mages.