Jose wrote:

It has nothing to do with the audio panel. There is a "transmit
interconnect" wire between radios. It doesn't always get installed. I
had to have one retrofitted after completion of my Garmin 430---King
KX-165A installation.


With such an installation, wouldn't each pilot be frustrated while the
other pilot is transmitting, defeating much of the purpose of the split?

No, it works just fine. You can hear a little garbling if you are
talking on close frequencies like 122.75 and 122.8 The Garmin audio
panel does not shut down the receiver of the radio not being transmitted on.