Some backing tracks are protected and won't open in audacity. If you select the relevant input option and press record and then play the track , audacity will record it. I'm not sure if there's a loss of quality but on my gear it's not noticeable.
After splitting the track and deleting the one you don't want, there's an option in edit to duplicate the track you do want. You can then make the two tracks stereo again. Before you export it you can adjust the output volume.