Okay, here’s your answer
1. Use the schematic in post #9, but with the corrections of post #10, e.g., change ‘com power’ to ‘mike’, change ‘com1 pwr’ to ‘com 1 mike input’, ‘com2 pwr’ to ‘com 2 mike input’. This will allow you to select com 1 or (but never both) com 2 for transmitting.
2. Receiving. The issue is, depending on the actual radios, one radio may ‘fight’ (try to cancel) the other radios’ headphone signal. Do not simply parallel the two headphone signals. Instead:
(A) method A. Insert a resistor in series with the wire out of com 1 going to the headphone jacks. Do the same with the wire from com2 to the headphone jacks. Any value of resistor between 220 to 470 ohms should work okay (but use the same value for both). You will notice that, due to these resistors, you will have to turn up the volume some, but most radios have volume to spare.
(B) method B. If you have more than 2 or 3 audio signals to hear (warning tones from autopilot? Traffic warnings from adsb? Warnings from efis?) adding series resistors to all lines will start to drag the volume down, maybe so low you can’t hear it well. In this case you should insert an audio mixer between all the inputs, and your headphones. Such circuits are available commercially, pre built for around $100, or build your own for $5. Since the mixer involves electronics,, you might think of a way to mitigate its failure.
I suspect A would work for you. In flight, position the switch to com 1, adjust com 1 volume as needed. Set com 2 volume fully off. If com 1 fails, move the DPDT switch to com 2, and remember to turn up the com 2 volume. As a bonus, in flight, if you wish to stay on ATC or tower frequency while getting ATIS information, just put the ATIS frequency in com 2 and turn up the volume. You’ll hear both tower and atis. Or, keep com 2 on 121.5 with the volume up, just in case someone calls. Note, depending on how close your two antennas are, you may get an annoying ‘bleed thru’ if you transmit on com 1 with the com 2 volume turned up.