I have a bad audio tape that contains both sides of the tape recording
on one side. So when played you can hear both sides at the same time
with one track backwards.

It's probably a fine tape. You're just playing it on the wrong
machine. There are several different track formats some of which will
play back like this when the playback heads don't line up with the
recorded tracks. That's where you need to solve the problem and it may
involve looking around for someone to either do the transfer for you
or, if you have a large number of tapes in that format, loan, rent, or
sell you the correct machine.

To put it a bit more directly... You're playing a half-track tape on a
full-track machine.