With the help of the Warcraft UI Forum, I found this macro:

/tar [target=focus, nodead] focus
/stopmacro [target=focus, exists, nodead]
/assist MasterToon
/focus

Having this on the slaves.
If they already have a focus, it will target the focus
If they do not have a focus, the Master's target becomes their focus.

You could have this focus macro on each of your slaves, in a slightly different position.
For example, Numpad 1 (Slave 1), Numpad 2 (Slave 2), and Numpad 3 (Slave 3).
So mouse-over a toon and press Numpad 1, a different toon and press Numpad 2.
Then have a Hex macro on say Numpad 4, 5, 6. As in immediately above the focus macro. This could be sheep, banish, fear etc.

So once the focus is set for a slave, that slave can easily find the focus target on a keypress (Numpad 1 for example) and easily cast a cc or interrupt (Numpad 4 for example).

I'm not sure how practical this would be in arena...
Have Numpad 1 make Toon 1 and 2 set the target as focus.
Have Numpad 4 be a cast sequence for Pass/Hex and Hex/Pass.
Similarly Numpad 2 can be focus for Toon's 3 and 4.
Then you could simultaenously Hex two of their healers as you burn someone.
If they trinket out of it, you can immediately re-Hex them.
Even if the Hex is not a CC, it can be an interrupt or a Wind/Earth Shock.