if you are using one computer to process both videos and send hdmi to 2 tvs
unless you have some sort of aftermarket special sound card, or multiple sound cards, im pretty sure the audio will be combined no matter what.
one of the things i dont like about windows is all the sound is together and cant be separated,
you can only change the output device, not change audio going to different output devices.
maybe someone will come along with a simple solution but thats the way i have always known it to be.