I like the idea of multiple Disc priests in a battleground, because of Mass Dispel, multiple mana burns, and because they can probably put out more punishment then any other healing spec'd healer. Also an extra AoE fear per priest.

But... if they are part of a team, instead of the entirety of the team, one gets you 80% of the benefit, which means a second healer would probably add a lot more.

I would think Disc (Priest) + Resto (Druid) would be fairly nasty. You have the mitigation and prevention of the Priest, along with the large burst heals. Combined with the rolling HOTs of the druid. Wild Growth and Holy Nova are both strong AoE heals, although WG is targetable while HN is centered on the priest.