Amaret's point is actually pretty valid, the ideal mixmax solution is for the SCH to apply dots, then take over whilst the WHM applies a set as well. That will give you the highest combination of overall DPS between the two healers.

However it takes a level of experience and coordination with your fellow healer that not many will attain for whatever reason.

Thus whilst not being quite as optimal, I tend to lean towards the safest option of just pushing towards solo healing the fight with the SCH DPSing near full time and pitching in with Galvs and Whispering Dawns at convenient times.