Two independent Big Sur installations on the same device Assume I have a MacBook Pro 2019 with Big Sur installed.
My goal is to have two different partitions on the same physical SSD.
I want to have two separate installations of Big Sur.
My question is:
How can I make sure, that nothing I do on either of the partitions has an effect on the other partition? Assume I install some software with admin rights on partition B. Can it be made sure, that the program installed on B has no possibility to read or manipulate any data from partition A, for example system files?
The answer is no.  Assume I install some software with admin rights on partition B. It can not be made sure, that the program installed on B has no possibility to ask the diskutil to remove entirely partition A.
Update
Maybe this explanation will make more sense to you. Originally with OS X, many problems could be fixed by booting to the OS X installation DVD. Security existed by storing the DVD somewhere safe. After Snow Leopard, Apple stopped issuing DVDs and instead opted for software downloads. A recovery volume was added to take the place of using the DVD to fix problems. The recovery volume contained limited functionality compared to a full OS X operating system. However, when booted to the recovery volume, changes could be made that are not normally possible when booted to OS X. This concept of recovery has been refined with each new release up to and including the release of macOS Big Sur. Along the way, Apple has taken many steps to prevent booting to recovery without first requiring user validation.
Installing a second Big Sur in a different partition creates in effect an extremely powerful version of recovery. So when booted to the second Big Sur, you are going to be able to mess with the first Big Sur, just as if you had booted to recovery. For example, SIP only applies to the partition of the currently booted Big Sur. The files in the other Big Sur are no longer protected by SIP. The same is true, if you were to install, then boot to Windows or Linux on your Mac.
The best way to protect your Mac is to only have on operation system installed. Once you install a second operating system,  you open up security vulnerabilities that Apple has not begun to address.