If you're using a party leader system, then switch to another window and have the new toon the leader on the switch.
Ditto for a Focus based approach.

With FTL, switch to any window, and the others assist based on where the command came from.

IS Boxer is the only software program, which does FTL for you.
You can drag Lightning Bolt, Fireball, Frostbolt and Shadow Bolt to button 1, on four toons.
No matter which one you are on, when you press 1, the other three auto assist you and all cast the spell on "1".
You can switch toons and within a second (double your latency actually; all boxing hardware or software has this same latency issue), the others know who to assist.
And this is straight spells from the spell book, no macro.