A question that emerges with discipline priests, moreso than any other healing class-spec I would say, is whether you can viably accommodate two of them in a ten man raid healing team. The reason this question is especially prominent for the discipline flavor of priests is because they are seen as reliant on a very particular spell: Power Word: Shield.
There is no question that Power Word: Shield is an iconic spell of the specialization and, most would agree, the spell that the spec revolves around.
What makes this tricky when compared to the central spells of other healers is that Power Word: Shield doesn't play nicely with another priest's Power Word: Shield because you cannot put more than one Power Word: Shield on a single target. Two Wild Growths work fine together, ditto two Renews, two Divine Lights, two Chain Heals, etc. Other healing class-specs largely focus on healing spells and healing spells almost always stack (as they must, since multiple healers often heal the same target). Power Word: Shield is something of an unusual mechanic in that it's a healing spell that doesn't heal.
This means that, at least on a superficial level, more than one discipline priest in a single raid group will be competing for shield targets. If one discipline priest shields a target, another one cannot shield that target. Each discipline priest, therefore, cannot operate with a free hand.
Or So It Would Seem
This would have been the beginning and end of the analysis in the latter stages of Wrath. Discipline priests were bubble-bots first and foremost. Played properly, their first duty was to keep everyone shielded whenever possible (whenever that pesky Weakened Soul wasn't on a target) and only secondarily (in ten man raids) heal raid members.
In such an environment, each discipline priest in a ten man raiding environment would be handicapping any other, as both couldn't be casting Power Word: Shield on every target (this was not so much of an issue in twenty five man raiding groups, since there were so many targets, two discipline priests could viably split up targets to shield). That meant that one or both discipline priests would spend quite a lot more time casting direct healing spells. That wasn't disaster, but it wasn't what discipline priests did best, so it was at least a mild nerf to one or both priests.
Cataclysm, however, changed the picture considerably. It did that in a few ways, but the net effect can be boiled down to this: Blizzard made discipline priests much less reliant on Power Word: Shield to be solid contributors to a healing team.
Let us count the ways in which Cataclysm gave discipline priests alternative casting patterns (spoiler alert: the number of ways is two.)
The Subjugation Of Tank Healers By Holy Paladins Has Been Broken
Time was, the tank healing community resembled an Old West saloon. You had your standard allotment of cow pokes, card sharps and...stable hands (I guess?) who got the job done, in a manner that was not flashy or spectacular. Quite workmanlike. Then a shadow fell across the room as a man in black (or white) swaggered in, a pistol on each hip. He looked around with a sneer and everyone who looked up to see who had walked through the swinging doors then quickly looked away, unwilling to meet his gaze. This fellow's superiority was palpable, it was unquestioned.
That gun-slinger was the holy paladin. The rest were discipline priests or restoration shamans. You could use another class to do your tank healing, but you only did so if you simply had no holy paladin available. It was sub-optimal...very sub-optimal.
That's changed. Holy paladins are still capable of healing tanks very well (and not quite so good on the raid), but all the other healing class-specs have become viable tank healers and discipline priests have really seen the biggest jump in this respect. It's no longer sub-optimal to put a discipline priest in charge of keeping your tank(s) alive. The interplay between talents like Renewed Hope, Divine Aegis and Inspiration, an interplay that was kicked up a notch by the patch 4.2 change that increased the output of critical heals, in addition to the interplay between direct heals like Greater Heal and Flash heal and the talent Strength of Soul made discipline priests extremely powerful in healing through major and continuous damage on specific targets.
Prayer Of Healing: Now With More Divine Aegis
Prayer of Healing used to be a spell that didn't have any special synergy with discipline talents. Discipline priests, when they weren't shielding, focused more on single-target spot healing. Prayer of Healing was more of a tool utilized by holy priests in their quest to AoE heal all the things.
Cataclysm has made Prayer of Healing a core discipline tool through a modification of the Divine Aegis talent. Divine Aegis, which has always (since it was introduced in Wrath) proc'd an absorption shield on the target of a critical heal, was now able to place automatic shields on every target healed by Prayer of Healing, critical heal or not. This was modified further to also proc extra shields on Prayer of Healing targets when the spell critically healed. Suddenly, Prayer of Healing had strong synergy with a core discipline talent...Prayer of Healing was now a vehicle to get non-trivial absorption shields out to lots of people, in a way that was spammable (as opposed to Power Word: Shield, which cannot be spammed on the same target due to Weakened Soul).
Combined with the actual healing that Prayer of Healing did, these absorption shields made Prayer of Healing a powerful raid healing tool for discipline priests, with a component of preventative medicine that has always been the discipline priest's stock-in-trade. Discipline priests could both use Prayer of Healing to heal the raid back up, or use it ahead of incoming raid damage to reduce the hit to health bars. It was most powerful when it could do both at the same time, with predictable waves of incoming raid damage.
Discipline priests can therefore settle into several roles. A Power Word: Shield-heavy role, a single target tank healing role or a Divine Aegis-powered raid healing role. By prioritizing different stats (mastery for heavy shielding, crit and haste for tank healing or Prayer of Healing raid healing) and minor changes to talents and glyphs, a discipline priest can play in a very different way from another discipline priest and still be equally powerful. No longer must multiple discipline priests step on one another's toes.
Indeed, discipline priest partnering with discipline priest is fine everywhere, not just in New York and Massachusetts.