I last worked on this stuff 2-5 years ago so I'm not completely clear. I'm probably very wrong on this nowadays so hopefully someone can give you better info. IIRC you can run two OSX guests on an OSX host with Apple hardware. Anything that isn't using OSX as the base OS is EULA violating. I see a lot of people doing it via esxi. I'm not sure if the EULA has changed and that's ok now or people are just doing it. Thankfully I'm out of that side of CI/CD and plan to never deal with it again.

Our initial goal was to have one OSX system running multiple xcode builds/tests. I think at some point we may have gotten it working by having different targets set up but I don't recall the specifics.

Sorry I can't be of more help. We worked with 3-4 other app developers and it seemed like all of us were doing completely different work arounds to reach the same goal. I'd love to know how King or any of the other huge (number of apps) developers deal with these things. It doesn't seem like it's talked about much.