Code:
 
/cast [target=focustarget, help] Heal 
/cast [target=focus, help] Heal
For my healing macros I add one more line

Code:
 
/cast [target=focustargettarget, help] Heal 
/cast [target=focustarget, help] Heal 
/cast [target=focus,help] Heal
This will check your main's target's target and heal them (basically whoever has aggro), if that fails it will check your focus target and heal (if friendly), finally if that fails it will heal your focus.