The toughest spec for a 2v2 warlock with a shadow priest, would be without a doubt 24/37/0 or a varient thereof. You have a spamable snare, instant corruption, Siphon Life/Soul Siphon/Fel Concentration from the affliction tree, and 15% stam/3% health/Demonic Aegis/Master Demonologist/Demonic Knowledge/Demonic Resilience/Soul Link.
It's tough. You're going to get focus-fired, so being tough is a good thing. It has a lot of goodies like Demonic Resilience (the rough equivalent of 120 resilience, minus the crit damage reduction). Run with a Felhunter to have a way to counterspell/eat debuffs.
It takes some getting used to but most warlocks in 2000+ ratings are specced something like this. Here's what I'd reccommend:
Linkie It lacks burst damage, but it's one of the hardest specs to beat in 2v2 arena simply because it's a ***** to kill.