This switch is for flaps, but your com could use the same.

I just wired two radios in a RV6 with a DPDT PTT select switch, which just switches the PTT lines for both radios. No audio panel. This same thing could be implemented with one momentary DPDT switch. No need to switch the mic lines, as the mic audio is split from the intercom going to both radios. Both radios should have transmit inhibit lines tied to each other which reduces the sensitivity of the non-transmitting radio while the other is transmitting.