I'm too lazy to look up the link, but Zan or Lax wrote a tutorial on setting up window regions where its a persistent "click through" to the slave windows of choice.

So, I set up a region on my main (Paladin) that passes clicks through to the exact same spot on just my priest's client.

I then set up healbot to occupy the same spot on both screens and reorder the party on my priest to match my Paladin.

I then disabled most of the casting clicks on my paladin window other than "shift click" which is cleanse. I then set "shift click" on my prest to nothing.

Voila.

When I left click a toon on my healbot with my main (paladin) my priest casts a flash heal on that toon.

When I middle click, my priest casts renew.

When I right click, my priest casts greater heal.

When I alt-click, my priest casts Circle of Healing.

etc.