I would make the mage main. Demonology locks are very easy to micro, and the pet does a lot of passive dmg (felguards at least) For mass aoe, you could go frost on the mage, and use a voidwalker tank, gather mobs, and blizzard away. Hell bring out the mages pet for more action. Dual specs give you a ton of options with a lock and a mage. I bet boxing a resto sham along with em sounds very tempting right about now