In my battlegroup the top 2s and 3s team are comprised of this spriest/mage pair that have single digit loss numbers. Mages are an amazing partner for you with the great control they have and they can peal a target off you quite easily. Any partner with good control is good for you. I run with a rogue and we do alright. Done some with a hunter buddy and that didn't work out well at all. As far as spell pen, you want to shoot for 240 if you want to be capped against mages. Resilience is a beyond important stat as you find you can take a pretty good beating (unless a melee like a warrior/druid get on ... hense needing a partner with great control and pealing abilities).
I play a shadow priest and do 2s with a resto shaman. We can usually outlast any double DPS combo since I can stop and heal the shaman if they zerg him or disperse if they zerg me. We can also usually beat any range/healer combo since the shaman can interrupt the healer while I dps down the range class
The problems we run into is with a Holy pally or resto shaman/warrior or deathknight combo. I do not do Arena alot but these combos give us the most problems since the pally can heal forever and dispel my dots leaving me with an insanely difficult warrior or deathknight to keep off me. In my opinion the only way our 2s team can beat these combos is if they are really bad.