How to setup two completely separate partitions on one SSD
No, since Apple designs the OS to be able to erase itself and undo any logical partition you make - anyone with moderate amount of time or technical ability can access the drive.
Your only play with shared storage is to trust encryption and separation of secrets. But that is flimsy protection if system B can always read, erase and modify system A storage. You don’t gain any data or interaction protection with two OS, you only gain complexity and two code bases and less protection overall.
The OS is always aware of the entire storage chain for all attached drives. You need external drives, two of them, to guarantee your “there is no interaction between the partitions possible” clause.
But, as long as you are on Catalina or higher, there’s never been a better time to dual boot two macOS systems on the same SSD. You can try it out and see if you can live with two OS and experiment if spotlight will find files and apps on the other system based on how you set things up. But this won’t make things more secure, it makes things less secure in several ways.