I think this fight is hard for healers because it requires a great synergy between the WHM and the SCH to handle everything correctly. It is very easy to overheal everything (but you get punished for that when the add pops in P2), and it is also very easy to miss a GCD, or forget a cooldown, and boom, someone from the group died...
I like reading guides for WHM and SCH, but discussing with your co-healer will help you a lot. You will both identify what to do, what not to do, when to use a CD (swiftcast, divine seal, Covenant, Whispering Dawn, Virus, Eye for an Eye...)
Personnaly, I improved my fairy control (Eos) and we get to the end of P2 more consistantly, while still DPSing and having more mana in the end.
My advice : identify why people die, ask yourself the good questions : how can I improve my gameplay ?