If we assume that it’s necessary to have two Roon accounts/licenses to keep completely separate libraries, it is also necessary to have two cores (two PCs)? Or is there a way whereby one machine can run Roon twice? I don’t mind paying two licenses, but having two core machines up and running seems wasteful.
There is no support for that use case. If you want to do that you have to tinker to make this work. Some of the possible issues you may run into: The standard installer may not let you choose where to install the files; Conflicts with network ports already used by the other instance of Roon Server, started first.
You may be able to circumvent these issues with virtualization of some sorts. Unfortunately, I don’t recall having read any instructions on how to do this here on the forum before, but there are threads about installing Roon Server in virtualization environments (IIRC: Docker, Proxmox).
Never considered doing it myself but an obvious option would be to run your examples of Roon in containers. But my guess is it is unlikely anyone is doing this so the only way to find out would be to try!
Take one of existing Roon docker containers and start two instances of them