Hey there using Windows 10, v. 2.3.3
When creating the first stereo or mono track and recording everything works great. When trying to record a second stereo or mono track in the same file it records completely flat and out of tune and it clips. At the moment only recording a single guitar track works. I’ve seen a couple fixes in here but none worked for me.
Any suggestions really appreciated,
Thank you 
and it clips.
That’s the phrase that pays. You are almost certainly recording your new work and re-recording the old work or backing track on top of it.
Make sure the Audacity recording and playback devices are real things and not “software devices” designed to make it possible for you to record internet music.
For example, that’s what the analog stereo connections on the back of my machine look like. They are real things and I can point to them. I also have a recording device called SoundFlower. That’s not a real thing. That’s software app designed to route sound channels inside the machine.
I can’t point to it or touch it.
Koz