I would like to know if there is a way to create two different systems/environments on your PC (Windows OS) that you will enter each by different username and password. When someone enters your computer he won't be able to detect there are 2 parallel systems.
Depends how much scrutiny you want to defend against. It would theoretically be possible to use full disk encryption with multiple partitions, decrypting each based on a distinct password. This falls apart if a user looks at the disk manager application in Windows though - they'd see partitions they couldn't use (but which they could reformat and take over the space of). It would also fail if they opened the computer, looked at the drive, and started wondering why they couldn't use all that space.
At a Windows level, there are various ways to determine whether there are other users, so you'd be able to see their presence, even if you can't access their data. These vary from names showing in quick access login screens, to being able to see the contents of the
c:\Users folder or the security permissions screens.