Are there any healing addons that can sort by health. If yes then i would use
repeater regions (or there equivalent if not using isboxer) and offset the healing add on positions.
So that when the main healer tarfgets the person with least health the slaves are targetting the next lowest and the one below that.

Really depends on a healing addon ordering by health.
Oh and the healing macros on all would have to work on mouseover and not assisting the main)

hopefully something like this is possible