Hmm,

You could try to use the following.

/cast [target=focus] Chain Lightning ; Chain Lightning

And for your focus macro have the main use a /clear focus

Example: If F2 switches to your second toon, have all other toons focus the second toon and have the second toon clear his focus. F3 third toons, etc, etc.

I think that might work. If the main doesn't have a focus it should cast a normal Chain Lightning then.