I think Rob nailed it. One of the primary reasons for a dual COM installation is for redundancy. The GTN 650 COM would continue to be fully functional.
In addition, if communication is lost or no G3X Touch displays are present, the GTR 20 will automatically tune the emergency frequency (121.500 MHz) and set the COM radio volume to a pre-determined level.
In my airplane, I tune both my GTR 20 and GTN 650 through the G3X Touch display because I like the user interface much better, but in the unlikely event of a display malfunction, you have still have GTN COM controls available on the GTN.
For anyone with multiple G3X Touch display installation, the GTR 20 COM can be tuned or controlled from any G3X Touch display.
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