It is not an "error", it is just a warning (that you can switch off in the "Warnings" Preferences) advising that you have more than one stereo track. Accordingly, your two stereo tracks (each stereo track in turn having two channels, left and right) will be mixed down into one stereo track when you export.
Audacity can only play back in mono (one channel) or stereo (two channels), so when you play your project in Audacity it is already mixing the tracks down to stereo. Similarly, the exported MP3 will be mixed down to stereo (one file containing left and right channels). The MP3 will have exactly the same balances you hear now when you press Play in Audacity, with the central overlapping part mixed together. The only difference will be the expected degradation in sound quality because you are exporting to a lossy format. If you want to see visually what the playback or export looks like mixed into one track, click Edit > Select > All, then Tracks > Mix and Render. You can always Edit > Undo it.
I would say though from looking at your screenshot, that the central overlapping part will distort. The volume of tracks when mixed together is always combined, and the lower of your two overlapping tracks is already almost at the maximum possible level of +/-1. I suggest you play that overlapping part and see if the green playback bars in
Meter Toolbar start showing red clipping lights to the right of the bars. If so, you have distortion. What you'll want to do is use
Envelope Tool to bring the volume of each track down in the area where they overlap.
If you actually want to export a four-channel file so that channels 3 and 4 are empty except at the overlap, it can be done if you export as WAV having set
Import / Export Preferences to "Use Custom Mix". If you export a six-channel WAV you can even convert it in other software to
MP3 Surround Sound (5:1), but I don't think from your description this is what you want.
Gale
