On Windows platforms, it seems either you can login as local user or as a remote user. Is there a way for multiple users to login to Windows? May be starting the telnet service can allow this? But this would be console based. How can each user access his account's desktop remotely?
Similarly on Linux, multiple users can do a telnet to the same machine but that is console based. Is there a way for each user to access the remote desktop?
You need Windows Server to create mutliple desktop sessions. In non-server Windows systems multiple desktop logon is blocked and only one session can be active. Logging on another user/session via Remote Desktop will lock first session (if second user have permissions to do it).
If you heard about/saw many active desktop sessions in non-server Windows - that was modified OS with swapped termsrv.dll. Licensing does not allow you to modify/swap system files and use non-server system that way and this is ILLEGAL.
Under linux this is possible, you just need to configure it. Unfortunately im not Linux specialist, i cant help. Maybe someone else.