Using Audacity 2.1.3 on Windows 10 home.
I have an audacity file that has two recordings mixed into one (stereo) track. Evidently the person who recorded the first session, forgot to save the file and then recorded the second session on top of the first session. My first question is how do I prevent this from happeningin the future? And, my second question is how can I untangle the two sessions?
Thanks for any advice you can give. 
recorded the second session on top of the first session.
Actually, if they pressed Stop and then Record, Audacity would have started a whole new, clean, independent recording underneath the first one.
When they stopped at the end of the second show, it’s possible to Save a Project and Audacity would have preserved both shows as independent stereo tracks.
Also at that point, the operator could have selected each of the two tracks one at a time and File > Export Selected > WAV, etc, etc and produced two perfect quality, independent stereo sound tracks.
What you can’t do is File > Export. That will smash both of the two shows together into one mix. Forever.
Audacity can’t split a mixed performance into individual voices, instruments or sounds.
This is a terrific training experience for the operator. If this is really a stereo track with both performances on both sides, that is the end of both shows. If these were career-ending shows, this is a good time to investigate double recording performances to produce backup copies.
Koz
There is one almost-zero possibility. If your Macs were set for Automatic Time Machine and Time Machine happened to kick in between shows one and two, then there might be a clean copy of show number one in the Time Machine Backup.
Have you ever used Time Machine to rescue content? This might be a good time to read up on it.
Koz
Thanks for your replies to my question. While losing the recording is not good, it is not critical. Just sad…
I will explain your suggestions to the operator so we can use this as a training experience.
Thanks again!