What, the majority of the damage in the fight is UNAVOIDABLE AoE:
Quake AoE
Interrupting Shout
Enfeebling Roar (also magnifying the rest)
Pulverize 1 and more relevantly 3
All of these abilities are entirely unavoidable. This means Quake Fire, Pulverize 2, Charge, and WW are the only avoidable raid mechanics and they occur in less frequency, 4 vs 5, then the raid wide AoE. This also assumes your raid perfectly handles quake fire, ER, and all parts of pulverize every single time, which rarely occurs.
These abilities cycle quite frequently (literally 60-90s). Assuming no overlap and casting 10 PWS per ability thats 50s with PERFECT timing and minimum GCD and only casting PWS, not doing anything else and we are talking about a literal hacked stats robot. In reality a single Disc priest, not close to GCD or a robot, shields ~ once per 2.2-2.5 secs at best over the course of an entire fight (butcher in this case so near ideal circumstances). For simplicity 10 shields/ 24s or at best half the raid per AoE event on Twins.
So 2 disc priest greatly reducing AoE damage for the entire raid on the fight, namely on ER and Pulverize where the damage matters/stacks with other abilities quite frequently. Pulverize, namely stacking with ER, AV, or Quake, is the real raid killer as its pure burst meaning effective health > healing, whereas Quake is easily healed over time with minimal threat to a player of dieing. On other fights where the AoE is lower damage or the entire raid isn't taking AoE every 30s, yes 2 disc is just pointless, but Twins is a near ideal fight for Double Disc priest, a class that is the best healer in the game anyway.