Hi, from what you’ve written I understand that both userA and userB are connecting remotely to the computer. Are user A and B sharing the same system account to get remote access? If they are, it’s perfectly normal that the session to the physical desktop is being migrated. NoMachine understands that the user is the same because the account is the same and therefore switches the NoMachine remote session to the other device.
If you don’t want this to happen, each user must have their own system account on the computer they want to get access to. In that case, with the free edition, if “usernameA” is remotely connected to the computer and “usernameB” tries to connect as well, username B can be refused by userA or userA can decide to disconnect. With Enterprise Desktop, on the other hand, if user B connects whilst user A is remotely connected, NoMachine asks user A what they would like to do (authorise to share or refuse).