Newguest“ Does "I heard them in stereo working together" mean that I heard them (guitarist, bassist) playing together at the same time in two different rooms?”
Sorry. The simple answer is "yes, and no." It all depends on your understanding of "stereo." Before I was born, "stereo" was applied to a primitive form of 3-D. Modern stereo is 3-D for the ear.
A device called the "stereopticon" used two cameras to simultaneously shoot still pictures from slightly different vantages. The device then allowed each eye to see only it's own picture, creating the illusion of depth. In the early 50's this was modernized as the "Viewmaster."
Is it possible to have stereo or 3-D without some kind of recording?? Again, it depends on what you mean. I've heard people say that a dog doesn't see in 3-D. What does that mean?
I think your use of "stereo" is a similarly poetic use. But you can't understand that if you don't know what "stereo" means.
Your author has already said that he heard them playing together at the same time in two different rooms. So of course that's what your sentence means. But that's not what "stereo" means.
We don't know if the two guys were working on the same song, or if they were synchronized in any way. It would be extremely unlikely. What they were doing may have happened to fit together for a short while.
In my honest opinion, the reason he brought "stereo" into the picture is that the experience reminded him in a poetic way of how in modern recording, tracks may be recorded separately and then put together to produce a stereo effect. The stereo effect in this case was unintentional.