main character targets the one that needs to be healed
lets say healing wave in is button 4 on all the characters
have the main use the actual spell
have the alts use a macro with this in it instead of the actual spell
[code:1] /cast [target=party1target] healing wave[/code:1]
press 4 and the alts targets wont change but they will cast on the main's target
or an alternative have
all the alts have slot 4 empty start to cast a detrimental spell hit your pause button have the main heal the one that needs healing and walk away clean

first one works better for me in instances because if i need to heal in an instance i need the charac ters full health