I checked on one of my system v2.0.0p1 and had no problem setting is not set and i can login in two different browsers and get no logout in the first one.
It’s strange
It get’s “better” if i activate this setting and logout with all users and try to login again it means another session is running 
That means you can only login one time and the next login is forbidden not the first one is logged out.
From the inline help
Normally a user can login to the GUI from unlimited number of clients at the same time. If you want to enforce your users to be able to login only once (from one client which means device and browser), you can enable this option. When the user logs out or is inactive for the configured amount of time, the session is invalidated automatically and the user has to log in again from the current or another device.
It is not working that the session is invalidated at logout. Or it is not invalidated completely. Your own session is invalid but you cannot login for the time defined as inactive time for this rule.