Not sure if it will help you with your setup but may give you ideas to try? I use 5.1 speakers and a small usb soundbar.
In Windows, 5.1 is set as default sound device and the sound bar is set as default communicaton device.
In msfs under sound there is an option to set atc on the default comms device.
If your headset is listed in windows as 2 devices, can you change or set them differently as defaults?