I have had my suspicion about this for a while but after playng the campaign again i think its clear it works
Normally healing is a very relaxed procedure as you just allow the priest to auto heal whatever is close by
Multiple abiuns can be tasked to heal a single unit but this is considered to be special to them
Well guess not
If you take 2-3 healers (seems to be the max) and micro them to heal a single unit they also stack heal at a faster but diminishing rate.
Make of that what you will