Or you can just set up your spell macros like I have to allow for any char to function as main or slave depending on if they have a focus or not.

/cast [target=focustarget,exists][target=target,exists][] Lightning Bolt

And the way my focus is set up, is that all the slaves focus the main, and the main has no focus.
When I change main, the slave that becomes the main clears its focus, and the rest focus that character.

For spells like CC where you want to designate targets for each character separate from the focustarget, just use regular target for that and change the macro to cast on target first if there is one:

/cast [target=target,exists][target=focustarget,exists][] Polymorph

Stuff like that, whichever way you do it, it works pretty well as long as you keep the targeting under control.