If User A connects, and then another user WITH the same account connects, the session will migrate. NoMachine understands that the owner is connecting from another client and thinks it’s the same person.
Is it not possible for you to allocate an an individual system account to the users which need to connect? The default behaviour is that if UserA is connected to the remote computer, and UserB tries to connect, UserA will see a pop-up notifying them of the attempt. Only if UserA ignores the notification, they will be bounced off.