FWIW, when the com is activated between two Sena 10's either can activate, but, it takes several seconds and sometimes more than one try to get it hooked up. That's why we just left ours on all the time. It does mean things like GPS directions and music do not work at the same time, but, a phone call will interrupt.
That is one of the things the 20 is supposed to allow is more than one function at the same time, but, I don't know how that works.