If you want to go by order within the group, that is a leader based system.
Party1, Party2, Party3 and Party4 are the exact syntax used in a Leader based assist.

This is only going to work when you're controlling everyone in the party.
Meaning, it is broken and not working in LFR and Battlegrounds and anytime you have a friend play with some of your toons.

You might be able to design a system like this, but the question becomes why?
Why do you want to design a heal based system, that will break anytime the group is not composed entirely of your characters.



An addon like Grid (with Grid Custom Layouts) or Healbot or Vuhdo (both are stand alone addons) gets you a fixed order sort.
As in the top frame is the same character on every toon, as is the second frame... all the way down to the last frame.



Here is how the party leader assist system works...
If you want to do a party order based system, maybe you can adapt this.

Everyone has base spells on Action Bar 2.
Everyone has /cast [@Party1target] Whatever... on the same keybind but on the second action bar.

So if you are the leader, #1 is Lightning Bolt.
Everyone else is on Bar 2, where it is /cast [@Party1Target] Lightning Bolt

Then you have a toggle for each lead switch.
When you push Hotkey 1, that switches Toon A to Action bar 1, and everyone else to Action bar 2.
Meaning one macro per the number of toons, on each toon on your team (with five toons on the team, everyone has five macros).

The format for changing the action bars is /changeactionbar 1 or /changeactionbar 2.

This accomplishes... the Party Leader does not use Party1 (because that is their F2 target, not themselves).
While everyone else uses Party1 which is the current leader...