takes a few more macro's but with my g15 I have it set up like this...

Main Healer:
/stopcasting [nomod]
/cast [target=tank, nomod]

Back Up healer:
/stopcasting [mod:CTRL]
/cast [target=tank, mod:CTRL]


Second Back Up healer:
/stopcasting [mod:ALT]
/cast [target=tank, mod:ALT]

Then of course I make a separate macro for each character. on my g13 - g18 keys is where I have my healing macro's set up in the same pattern as my screens.
This requires a bit more action for healing, but also allows me to cast heal with more then one character and will allow me to use a back up healer if my main healer is oom.
This also allows me to spam my dps button on all the healers and interrupt for heals as needed.