[Someone]

If you are “using macOS High Sierra or later on an APFS-formatted startup disk”, you can create additional APFS volumes on the startup disk and install the beta there.

That way, you can switch between your ‘normal’ OS and the beta, and reverting is a matter of deleting the volume you created for the beta (https://support.apple.com/en-ca/HT208891)

Of course, that is riskier than installing on a separate disk or even Mac, as the beta PS could easily damage both volumes.