I pvp with a shadowpriest on my lock. Frankly I am stronger at my class than he is on his priest (his main is a rogue). Anyways mainly I'm wondering when do shadowpriests shift in or out of shadowform at the higher levels of 2v2 play? Are you in shadowform 100% of the time and just using shield? Do you never use shadowform at all so you can heal? Should you be shifting in and out a lot to heal? Any help would be apreciated.

From my experience in 2v2 (as paladin/warrior) if we can get the shadow priest to drop shadowform we've probably won, because we have the initiative then (i.e. the priest is reacting to our damage). As a 2-DPS team, you have to drop someone quickly.

What Thelyna described is true when you are facing the usal warrior/healer Team. Your goal is to drop the warrior as fast as possible while CCing the healer. If you don't succeed to kill the DPS, you'll lose. The simple fact that a shadow priest will be out of mana in under one minute, healing or not, will be the factor that you center your strategy around. And once he has to heal, you won't be able accomplish this before your priest is dry.

But, there are diffrent teams where you have to heal. In the lower ratings you'll often face Rogue/DD Teams wich rely on their initial burst and crowd control, just as you do. A good example is rogue and feral druid. They have to kill you before their fear counters (COS, PvPTrinket) are used or they will be killed only by dots. Our usual tactic is for me (priest) to stay alive as long as possible while fearing, while my wl is dotting them (especially the druid) up and stays at ~36y range (or out of blind range) once he used his mass fear, throwing in coassional fears/deathcoils to minimize the damage they can put out. Sometimes I'll die, but the time it took them to kill the me put them into a huge disadvantage and we win anway.

The only time it is a good idea to drop shadowform in my experience is vs 2x dps teams that don't snare you, like mage/mage, warlock/shadowpriest, etc. It's especially a good idea vs warlock teams that try and double-dot, because while shadowpriest heals are usually pretty underpowered, mending will still bounce a full 5 times every 10 seconds. Otherwise, if you are in a situation where you need to drop and the other DPS isn't dead, you're 9/10ths of the time going to lose. If it's healer/DPS, the heal-specced healer will win the mana war 110% of the time, shadowpriests have enough mana problems alone just doing dps without spamming flash on themselves.

That being said, this macro may be of some help:
/cancelaura Shadowform;
/cast [target=target] Prayer of Mending;
/cast [target=player] Prayer of Mending;

just replace prayer of mending with that and hammer it if you're in trouble, it'll drop shadowform without having to click it off.

I almost always drop shadowform when I'm up against casters -- the 15% extra damage probably isn't going to win the game for me, but just prayer of mending alone is worth it.

Otherwise I rarely drop it and it's highly situational when I do. It CAN work, i.e. if you get your opponents feared and you have a few seconds to regroup before they come back in and your warlock friend is sitting at 3k hp with healthstone already gone, but again -- highly situational.

I've played a few matches with a shadow priest and if the situation forces him to switch it off, it's generally a bad sign.

Focusing on one target is the way to go, especially if your UA specced. Nothing survives massive dotting from both a priest and a warlock.

When you've applied every dot on a target, just fear and nuke the other one.

I 2v2 with a shadowpriest, and I'm in the same boat as you..he's a bit undergeared. It really depends on what classes you're up against. Melee's are usually not worth dropping shadow for. If they're composed of only ranged dps, it's worth it if he can LoS kite. Otherwise you just need to burst down your intial target..and choosing that target is the key to winning. Sorry I don't really feel like getting into target choice ..it's complicated =]

shield if needed, but your priority is NUKE SOMEONE DOWN before u shift out. more often than not if you're forced to shift out before 1 opponent dies, u lose

The shadow priest that i always roll with on my warlock never starts in shadowform. He says it is too much of a target on his back, so he lets me start taking the hits first, and keeps up with the PoM.

In my lock's 2v2 team, my shadow priest friend starts the match out of shadowform and then goes SF after our first kill... I'm certain this is one of the reasons we are crappy.