Ok - so I got a chance to play around with this and I got it to
almostwork as suggested by Dreamkeeper's last paragraph.
So I went into the ACT options for the PCR, and made sure that button A8 was excluded from ACT so that I could assign a note to it - the controller 80 value that's the default there, at least in the map that is loaded here (0) outside of the DNAP mode, was not working in that it only worked while I held down the button. So I assigned a note to it and set the two buses enables to the same note number as a trigger, and started them opposite-selected of one another.
When I click between the two sends turning one off and the other on, the sound goes from bus to bus as I would expect, but when I hit the A8 (toggle) button, the lights on the tracks SHOW that the buses have switched, but nothing happens with the routing of the sound on the bus.
What the?