You may know that to disable the ability of multiuser login under the same user name at the same time you enable this parameter:
login/disable_multi_gui_login 1 (to block multiple logins), 0 to allow multiple logins
But here is one you might not know about. Enable users who need multi logon capabilities by enabling this parameter:
login/multi_logon_users separated by comma and no spaces and id name to allow certain users multi logins.