The reason there are different plugins is that the two users have different configuration data, by default it is stored at the users %APPDATA% location (normally C:\Users\{username}\AppData\Roaming) in the subdirectory
calibre.
Task Manager will show you the disk location of an executing program, right click program name in the Processes tab and select Properties.
If you want the users to share the configuration data you should move one (which ever is the 'better') of them to a common location. I suggest the same location as the libraries. Then for each user you'll need to set the CALIBRE_CONFIG_DIRECTORY
Environment variable to reference that location.
BR