I am not sure..but i think you turn off Fast user switching,because it is a functionality on modern multi-user operating systems which allows multiple user accounts to log on to a computer simultaneously and then quickly switch between them without quitting applications and logging out.
The reason for turned off Fast user switching is
First, this could be a security policy requirement. A security requirement might be that a user should completely quit all applications and log off from the computer after finishing his or her work on the computer.
By disabling the fast user switching feature, you hide the Switch user button in the Logon user interface, in the Start menu, and in the Task Manager.
Another reason could be performance issues. The fast user switching feature uses some system resources which can be freed in case the fast user switching functionality is not needed.
The above reason you tried to log into another account but you got the message
logon denied only one user session is allowed