Multiple Tracks, Swap channels

I have a nature video with an original audio track, left and right channel. I wanted to add a music track to augment the original audio. I opened the video in VLC by choosing the Media tab/ Open Multiple Files. I added the original video with the nature sounds. I then ticked the “Show more options” selection and the “Play another media synchronously” option. I then added and selected the music file which i wanted to play along with the original audio. Returning to the first Open Media window I chose Play and then went to the Audio tab and chose Audio Track 2. I was then able to save the Video file, which now had 2 audio tracks, each with a left and right channel.
I was able to play the file on my TV, which allowed me a choice of which audio track I wanted with the video, either the original natural sounds or the music background.
But what i want is a track with the left channel of the original natural sounds track replaced with the left channel of the music track so when played both sounds are audible.
Is that possible with Audacity? If so any help is greatly appreciated.
I should also say when i open the file in Audacity it asks which Audio Track I want. One is Index[00] Codec[mp3], Language[eng], Bitrate[127597], Channels[2], Duration[1738] and the other is Index[01] Codec[mp2], Language[eng], Bitrate[127706], Channels[2], Duration[1739]