Not exactly what you want, but I do this:

On your main, use numpad 1-9 to setup raid targets (in your key bindings setup).
On your alt 1, bind numpad 1 to /assist <Mains Name> /focus
On your alt 2, bind numpad 2 to /assist <Mains Name> /focus


Now you can (if you ahve decent party frames that show party targets), easily see if they a) assisted correctly even after lag, and b) know which is focused.