G15 or Xkeys might come in handy with a the ingame g15 addon

There is a /target party1
/target party2
/target party3

And so on
So what you do is bind the following underlined keys if you are using g15 to the following using ingame macros, and binding the keys in the ingame keybindings if you use with the g15 addon.

By using the

/target party#number
/cast (insert spell here

you avoid confusion and its also handy for when you aren't multiboxing as well if you are a raid healer because no matter who or how ur party is setup it will be a dynamic method of management.

0 0 0 /target party1
0 0 0 /target party2
0 0 0 /target party3

0 0 0 /target party4
0 0 0
0 0 0

0 0 0
0 0 0
0 0 0