Would never have happened. There were far too many protected technologies, weapons technologies, involved in getting to and from the moon. At some point the plug would have been pulled.
The 1975 flight did happen, and before that extensive familiarization with each other technologies needed for that. So I don't think it that obviously would never happen; however, historically speaking we have what we have...
Two ships meeting in space is altogether different than two teams working on a single rocket/program to the moon. Apollo-Soyus involved learning of each others abilities, a critical part of cold war/mad deterrence. It did not involve sharing how those abilities were created. The americans didn't give the russians a copy of the guidance computer.
> It did not involve sharing how those abilities were created.
Same approach could be used for cooperative Moon flights.
In fact, after Apollo-Soyuz we've had Shuttle-Mir and then ISS, and there are plenty of cooperation and cross-studying in those projects. From some point of view, technologies on both sides of Atlantic were similar - there were exceptions, of course, but for professionals working on projects for years the inevitable knowledge osmosis tells a lot.
In another sensitive area, achievements in thermonuclear studies, cooperation did happen.
