I have an accompaniment track that is SPLIT-TRACK. The music is in the TOP and the background vocals are on the BOTTOM. HOW do I MERGE the two together.(so the vocals are on the same track as the music) … THEN How do I make it STEREO again? Is this even possible?
THANKS! 
Adjust the track “Pan” so that the tracks are positioned left / right as you want them.
If you’re ready to export, just export and they will be mixed automatically.
If you want to continue working on them, you can either keep them as separate track, or mix them together into a single track (See “Mix and Render” https://manual.audacityteam.org/man/tracks_menu_mix.html) - note that once you have mixed them together, you will not be able to “un-mix” them.
If it’s not actually split into two separate tracks you’ll have to split it. Click the little drop-down arrow to the left of the waveform. If you choose Split Stereo to Mono you’ll get two mono tracks that will be automatically mixed to a single mono track when you export.
THEN How do I make it STEREO again? Is this even possible?
Not really… You could choose Split Stereo Track which allows you to edit left & right independently. Also to the left of the waveform there is a left-right pan slider for each track so you can blend the two tracks while leaving the vocals slightly to one side and the instruments slightly the other way. But that’s not really “natural” or “normal”. Usually the main vocals are in the center with the various instruments spread left to right across the soundstage. You can’t do that with all of the instruments pre-mixed.
There are “simulated stereo” effects such as [u]this one[/u]. You might try that on the instruments, leaving the vocals in the center. (I’ve never used this, but you’ll probably need to make separate vocal & instrument files and mix them later.)
There are other 3rd-party plug-ins, or I’ve done it by using different/complementary EQ on the left & right channels. EQ gets a little tricky because +6dB (double the signal) on one side is NOT offset by -6dB (half the amplitude) on the other side. When I did it I made a spreadsheet for the dB calculations.
Assuming one track has only music and the other only
vocals - try this. Look for the slider on each track that has L - R. Move one 60% left and the other 60% right.
Give the results a judicial hearing. Adjust as necessary by pushing sliders either greater than 60% or lesser.
When done export as WAV.
Making back-ups as you progress through a project.
Firstly, export your two tracks as they are in Audacity’s own format (FILE / SAVE PROJECT / SAVE PROJECT AS…) This forms an original archive copy. Belt and braces, you can also archive it as a WAV. (FILE / EXPORT / EXPORT AS WAV) This forms an original archive back-up copy.
Save your mixed copy in Audacity’s own format (as above) with a different file name. This is back-up copy and forms an archive copy as well.
In addition, export your mixed version as a wav (as above).
All subsequent changes always do a SAVE AS… with an incremental file name.
Always make continual copies of your project as-you-go. Didn’t read this in a forum, website or book, just found out the hard way.
Forgot to add that the exported wav file will be in stereo.