Disc / Holy priests would offer some synergy, but they offer fantastic big heals (greater Heal and Pennance) which often don't need to overlap and if both cast on a single target would result in a huge overheal which you don't need.

Druid Hots can be rolled on multiple targets as required, and 2 resto druids can heal pretty much any BG force with gear, the wild regrowth is amazing and it doesn't require huge amounts of thinking and micro-managing 2 different sets of abilities. Cooldowns can be managed easily with macros (swiftmend and Nature's Swiftness) and if you BG you can tell how powerful even one resto druid is with its own hots, let alone multiple ones.

Also with druids, you could easily dual-spec moonkin / feral etc with the gear to change the dynamic (ie: druid tank and healer for AV bosses etc) A moonkiin spec could easily use the same gear as a resto, and would provide options, shadow priests in their current form are less than ideal DPS in BGs and even though I have my team at 80, I rarely use them as anything other than PvE healers.