Well, you need the person who has the focus to target their focus.
From there, you can assist them to also target their focus.

For heals, I really like this structure.

#show Lesser Healing Wave
/assist Tank --- usually have this line for boxing ---
/cast [help] Lesser Healing Wave; [target=targettarget, help] Lesser Healing Wave

So if whatever your active toon (usually the tank) is targeting is friendly, that gets the heal.
If the tank's target is hostile, ie the mob you're trying to kill, the target instead hits the hostile's target.

Almost all the time.. the set up is:
Tank attacks --- > Mob
Mob attack --- > Tank

So your heal goes to the tank.
But if the mob switches to someone who overnuked, your same heal key goes to whoever has the mobs attention.
Which is usually the person who needs the heal the most.