I'm running this exact combination right now. They're all level 57.

Priests are specced heavy shadow and Warlocks are specced affliction. I try to get as much synergy as possible on the shadow damage. One priest is specced for misery (+5% spell dmg), and both have shadow weaving. One warlock is also specced for Malediction (13% increased shadow damage with curse of shadows) and uses that while the other warlock uses curse of agony. The damage boost is pretty significant for all 4 characters when this stuff stacks.

Of course my main goal is PVP, but it seems to work nicely in pve as well (just questing now). With all the dots ticking my group can handle a lot. 2x vampiric embrace really helps keep the hp up. Last night I ran into qual'tharas (or whatever) in EPL at level 56 and fought of 7 level 60 mobs without a sweat. Dot em up and round robin fear bombs.

I think at 70 I will probably get a paladin or warrior to tank instances. I don't know that it's possible to do heroics with voidwalker or felguard tanks...

Oh, and at 55 the group killed a 70 shaman in season 2. And I'm still SM gear and lvl 30 greens...haha. I was proud of that