Unless I'm misunderstanding what you're intending to do here, I don't think Rx would be my first choice of program.
It's more about audio restoration (getting rid of crackes and hums etc) whereas I think you need something with the ability to change the volume, eq etc of the two stereo tracks and then render them to a single stereo track.
I use Sony Vegas for this but it sounds like Audacity would do it too.
I would use Rx for taming any coughs, squeaks or hums you might have in either recording and definitely for the final resampling and dithering of your mixed file.
Apologies if I'm getting the wrong end of the stick here.