You have a main toon... named Zantos.

You have one or more alternates, names A, B, C etc.
The macro's on A, B, C etc.. could read.

#show Lightning Bolt
/assist Zantos
/cast Lightning Bolt

This works, as long as Zantos is around to select targets.

Probably the simplest assist method is to use Focus.

Everyone's spells are:

#show Lightning Bolt
/cast [target=FocusTarget] Lightning Bolt

Or...

#show Fireball
/cast [target=FocusTarget] Fireball

Then you need one hotkey per toon on your team.
So on a three box team, three hotkeys.
Or five hotkeys on a five box team.

Each of the hotkeys are on the same keybind.
So A, B, C, D and E all have.
/Focus A
On say Alt F8.

And they all have... /Focus B... on say Alt F9.

When you switch from one toon to another, press Alt F9.
Now B is the focus for every toon.
And every toon casts at the focuses target.
Which is some protection against having your first toon killed.