The system and data volumes in a volume group are connected to each other. You con't re-use one data volume with multiple system volumes, because each data volume can be in only one volume group. When I install multiple versions of the OS, I make three volumes, one for current shipping macOS, one for the beta, and one for documents and other user files. I call them "Current", "Next", and "Documents". All of the documents and projects I'm working on go on the Documents volume instead of in my home directory.
Although it's possible to move your entire home folder to the Documents volume using an advanced setting in System Preferences, I don't recommend it. Some apps like Mail use a different format for their database on disk in older and newer versions of macOS, so the old and new version can't both use the same database on disk. I end up storing my email twice, once on Current and once on Next, but to me that's better that the risk that I could corrupt my email database and lose messages.
