I've been playing mage/priest for quite a while now. I think the best mage comp is either mage/sham or mage/lock but mage/priest isn't all bad.
Setups with lock and druid are hardest IMO. Me and my partner don't even bother with hunters anymore.
In general vs healer+dps teams you cc the healer and nuke the dps. we just string full sheeps and fear and arcane torrent together to get kills. vs dual dps comps you should always keep in mind that melee classes are easier to cc cause you can use r1's and nova to fill the sheep dr space.
As mentioned before a mage alone can't get a kill off you need assistance from your partner. So my priest always plays very offensive.
Be prepared to lose a lot though, certain comps are simply impossible (hunt/dru, lock/dru,..) and a lot of other comps simply have the upperhand.