Tainter

Rogues can't shadow step to friendly targets.

Against two mages, or any two ranged dps you have to stick around line of sight obstacles and so does your partner. Agree on the target and try to dispel frost armor from that target. That makes it a lot easier for your Rogue and he will take less damage.

Fear water elementals, they can't do anything about it. If you can catch both and maybe even one of the mages then you'll take quite a lot less damage. Water elementals on the Priest is generally a bad choice, because then the Priest is essentially immune to polymorph.

Keep your instant heals up. Stay away from the Mages. Counter-spell has a shorter range than all your heals. When the Mage blinks or moves to you stop casting and move. If he chases you he doesn't do much damage to your Rogue. Tell your Rogue to disappear when you can't heal him. If you get polymorphed or counter-spelled and he's on 50% then only CloS and Vanish can save you.

Put Pain on your Rogue's target, the sooner it dies the better. Make your Rogue play defensively. Start with sap, use cooldowns to avoid damage, like shadow stepping behind a Mage that's casting a Frostbolt.

It's always like this in Healer 2v2:
If you or your partner die to burst then think of ways to avoid damage. There's always something both players can do. It's not solely the healers job to counter-act all the damage.