I went with 2 priests and 3 warlocks for this reason. I think there are a lot of ways these two classes can benefit each other, plus the ability to spec all 3 warlocks differently. Here's my ideas:

1. Curse of elements (w/ malediction, 13% more damage), shadow weaving (10% for the priests), misery (5% all spell damage). I think these effects will probably be the most significant synergy between the classes.
2. Free stuns (blackout)
3. Free group heals and mana from priests
4. Each warlock can use different curse
5. Possibly different specs for each warlock (1 picks up UA and shadow embrace for 5% reduced physical, main will be SL/SL meat shield, 1 with felguard for intercepts)