[Raiding Priests] How Many Disc Priests?

This evening after doing Wintergrasp on my Druid, I hopped onto my priest to do Vault. Now, I've been playing my priest for just over 2 years now, predominantly Holy, but recently I made the jump into discipline healing.

A few weeks ago in an Ulduar 10 achievement run a second priest was brought into the raid to assist with healing. The priest was discipline, and I noticed while meter watching (yes, I meter watch mainly to get a general idea of my performance, and to reaffirm that I am doing my job. A throwback to some abuse I sustained at the hands of an overzealous holy paladin.) I noticed that my effective healing and my effective absorbs had gone down drastically from fight to fight.

While thinking about it later on, I had noticed that my usual targets (namely tanks, and some DPS) were bubbled anytime I went to apply bubbles. This nerfed my overall absorbs.

I didn't really question it at that time, but figured that only one discipline priest would be effective in any raid environment.

Tonight really confirmed that for me. I headed into Vault. We had three healers, which usually is sufficiently overkill for Vault 10. The healer balance was off, causing extreme amounts of fail. There just wasn't any viable long range group healing.

After that raid fell apart, I did a little digging, because I had just been runninjg off a theory that you could really only have one disc priest in a raid. I found that you could have more than one, but you need to organize the healing to a point that it would only be good in a fight similar to Hard Mode Anub'arak. (Source)