You can set it up however you want.

Just the wizard gives a default set up.
Most will adapt from the default, in which case we have.

a) You press a key, from 1 to =, example "1".
b) The non-active windows assist the active window, acquiring its target.
c) All windows then receive the same key you pushed, example "1".

If you want the toons to target something different from what the active window is targeting at the time the button was pushed, it should be in their macros too.




You can set up sticky targeting, if you want the slaves to stay on something, even while changing targets on the main.

The easiest might be to disable the Mapped Key: FTL Assist Me.
Do this by creating another mapped key, with two steps.
Step 1 disables and Step 2 enables the mapped key.

There are probably better ways to do this, but that's a quick and easy method.
While the key is disabled, the 1 through = keys cannot have the slaves assist the main, so they stay on their current targets.

You could accomplish this entirely within wow, by having each /Focus their current target.
And giving their dps keys, [target=focus, exists] as the first condition.