It does not seem that my recently activated accounts are eligible for it, so anyone able to try out two or more Shadow Priests is very welcome to post a trip-report of what it's like.

Specifically, several max-level high-spelldamage MindFlay at the same time, Devouring Plague, Shadow Word: Pain stacking, etc. I'm regularly killing red-conned enemy players with my priest duo, was going to see what it would be like when much higher.