Ive run Belf Disc Priest / Disc Priest lots and its mint. You miss snare or stuns though so returns can be tough at times. But for controlling mid and capping/support its very powerful. My favorite healer Duo comp to run. Watching hunters try to backpeddle away while getting mind blast penance spammed to death is hilarious!
Holy/Disc Priest - Ran this for chastise. Overall inferior to Disc/Disc but still very playable.
Shadow/Disc Priest - Ran this for snares. Again inferior to Disc/Disc. Id say the weakest of the Priest/Priest combos.
Tauren Prot Pally / Belf Disc Priest is also amazing. War Stomp, Hoj, Fear, Arcane Torrent, and SoR will mow through multiple opposing healers. Still great ranged burst DPS and you add better sustained DPS with the pally. I run this with a lot of int gear on my pally. Great flag team and still very powerful at controlling mid. Better return capabilities then disc/disc.
.Holy Pal / Disc Priest - The best healing potential and still have hoj from the pally. A great combo but always feels a little too defensive for my play style. The burst from Disc/Disc allows you to CC and kill opponents quickly to even the odds fast when out numbered. Hpal/Dpriest tends to lead to long drawn out affairs
Resto Shaman / Disc Priest - Great flag running combo and heal synergy works great. Like Hpal/Dpriest it gets a little too defensive. In WSG many times your group flocks to you and there are no heals on offense. This results in a lot of turtle type games with no one getting returns. Better have some decent rogues on your team or get used to sitting in the base with the flag for long periods of time. Shaman adds mass utility though between totems, purge, and shear you can really frustrate opposing teams. Fun combo but I just prefer the more offensive capability double healer teams.
Anyway, thats my 2 cents on the teams Ive run quite a bit. Cant stand playing my druid as resto so havent run the Rdruid teams much. But the little that I did, cant say I liked it at all.