I’ve always done this using an external drive. I clone my Mac’s drive to the external, boot to the external, then download and install the beta OS on the external. When you first boot to the external, check the startup applications and remove them – you won’t want mail, for instance, going to the test partition! Also disable Time Machine and any other backup app.
By “always” I’ve done this with OS betas from Snow Leopard to Catalina.
I just finished installing Catalina on an external SSD (Samsung T5). After the installation the first 2 tries to restart resulted in the circle with line through it icon at startup, but it worked on the third go.
A couple of things to report so far.
My Catalina is installed on an external SSD. When this is connected via an OWC Dock, it wont boot - instead it gives the prohibited disk symbol (circle with line through). When connected directly to the MBP, it boots as expected.
Secondly, when an iPad is connected to the computer via the Finder there’s no way I could find to see the iPad’s serial number or device ID as required to register the iPad for the iPadOS beta, in either the Finder or Music app. However you can copy it from About This Mac / System Report / USB.
You don’t need to register the device for the beta, do you? Just download the image, connect the device to iTunes (Finder on Catalina?), Option-Click update or restore and select the image.