So you're 5-boxing, and you press a button to tell your healers to heal you?

I assume that this button in bind to a key press in your main.

Assuming you broadcast this key press to the 4 other wow clients.

You can just create 2 key maps, and each will do different things to each client.

Suppose I have 5 Paladins.
P1 is Main tank
P2, P3, P4 and P5 are Healers.

Key Map Ctrl+1 and Ctrl+2 broadcast from main to others.

P1.Macro
Ctrl+1 = /say Flash of Light me (or nothing)
Ctrl+2 = /say Holy Light (or nothing)

P2.Macro (same as P3, P4 and P5)
Ctrl+1 = /cast [target=party1] Flash of Light
Ctrl+2 = /cast [target=party1] Holy Light

So now, when you want a heal in your main, you hit Ctrl+1 or Ctrl+2.

You can change those to other keys.