Round-robin, software side should be extremely easy to do this.
Set it to send the key bind, for your /Focus macro to one character at a time.

If you want to do it entirely within game, with something that does not support Round-Robin...
You could give each character their own keybind, and have one macro on each of the toons.

Probably easiest with one macro to set Focus.
And another to cast at your Focus, if you have one.
Something like:

/Stopcasting
/cast [@Focus,Exists,Harm] Hex