Last year, I was able to install the Catalina beta on a partition of my main MacBook Pro 15" 2016 Hard Drive, and use StartupDisk to effectively switch between the stable Mojave macOS and the beta Catalina based on these instructions from Apple. This year I attempted to do the same with Big Sur, but it is not working. Here are my steps:
In Disk Utility, I created a new partition of my drive (I have tried with APFS and APFS encrypted formats, both have failed)
I downloaded the macOSDeveloperBetaAccessUtility.dmg file from the Apple Developer beta download website.
I opened the disk image and opened the package inside and began to follow the instructions to install the Big Sur macOS profile, but when I got to the part of the processs where I could choose the Install location, it only showed Macintosh HD as a viable option, and my partition (which I named Big Sur) had an exclamation point reading "macOS Developer Beta Access Utility can't be installed on this disk. You can only install this software on the disk that is running macOS."
I think you have to create the partition, install an OS (most likely Catalina), log into that OS, and then run the Beta Access Utility. I don't think your current OS can install a dev profile into a separate OS instance.
After installing the profile, you should be able to then upgrade that OS to Big Sur.
After installing the profile, you should be able to then upgrade that OS to Big Sur.
