Well you could have a macro on each toon, with a different modifier per toon.


So your macro might be...

Toon A:
/stopcasting [mod:alt]
/castsequence whatever

Toon B:
/stopcasting [mod:ctrl]
/castsequence whatever

Not sure if this will get you what you want.
I use keyclone and it works reliably for me. I've never used anything but its broadcast/override keys.
When I want to round-robin, its always via warcraft macro's.