My current plane has an Apollo SL60. This radio, as well as other Apollo radios, allow you to monitor two frequencies with one being the primary frequency. I use it all the time. Is this feature found on the Garmins?
(I’m assuming you mean,eg, pressing the COM2 button to listen to that freq, while listening/transmitting on COM1.Even the ancient training Cessnas I used had that function. Do you mean something more than that?)
I think the new Apollo radios have a feature where a single COM radio can be transmitting and receiving on the active freq while also monitoring (i.e. receiving) on the standby frequency. This is not a function of the audio panel (i.e. not COM1/COM2 switch like you mention) but rather a feature of the COM radio itself.
The Garmins don’t do this. The COM portion of the Garmins is very much like just a regular flip/flop COM (with some extra features like being able to program the standy frequency from the GPS database, etc.) So the only way to listen to two frequencies at once is like Jim suggested, just using the audio panel to listen to the 2nd COM radio.