You can definitely go with mixed teams.

IS Boxer will essentially have all of the non-active windows, auto assist (via FTL) the active window.
If you push "1" (or 2, 3, 4, 5, 6, 7, 8, 9, 0, -, =) the other windows assist the main window, and then all windows receive that keybind.

You could put straight spells on "1", for five toons... going with say: Fireball, Lightning Bolt, Frost Bolt, Shadowbolt and Wrath.
From any window, pushing "1" will have them all cast those at the target of whoever is the main/active character.

You could also look at the Mercurio macro sticky or the Aragent IS Boxer Macro threads, and go with a castsequence macro of some sort.
So when you push "1", it is the equivalent of /castsequence Spell A, Spell B, Spell C, Spell D... etc, instead of a straight Cast This.



You could have your healer cast something round-robin.
If you adapt the FTL Target Me, to target only the healer (copy the mapped key, edit the copy) so you have Healer Target Me...
You could make that a five-step macro (as in a different thing occurs on each of five sequential presses, in order).
On each step the mapped key will have the healer target a different character and then cast Rejuvenation, or whatever spell you have set up.

Most of us will opt for Grid/Healbot/Vuhdo.
Set them up alphabetically, and position them so they're the same on every screen.
Then have a Repeater Region over the whole thing, so clicks on the frame are sent to the Healer's window -- no matter what window we happen to be in.
Then use Clique (if using Grid.... Healbot/Vuhdo don't need an additional addon) to interpret various click types as heal the target (whose name is in the Grid Frame) with this type of heal because I made that type of a click.