my 5 shaman team uses

/stopcasting
/cast [target=focustargettarget,help,nodead] [target=focustarget,help,nodead] [target=targettarget,help,nodead] [help,nodead] [target=player] Healing Wave

for healing on 6 keys. one for each shaman, and one for all of them at the same time.
targets will be:
1. the target of my focustarget, if friendly and alive
2. my focustarget, , if friendly and alive
3. the target of the target of the healing char, if friendly and alive
4. the target of the healing char, if friendly and alive
5. the healing char himself

this way the only choice to make is if the char in need of healing is the actual target of my focustarget, or not. if it is, i just press the healbutton of one or two other shamans, depending on how much heal is needed.
if it is not, i switch my focustarget for a second to the char in need of healing and press the according healbuttons.
no worrying about which button lets which toon heal whom.