How to Enable / Disable Multiple RDP Sessions in Windows 2012
How to Enable/Disable Multiple RDP Sessions in Windows 2012 By default, Windows 2012 servers allow a single Remote Desktop session. If only one session is available and you take over another person's live session, you may choose to enable multiple RDP sessions. This article describes the process for enabling and disabling multiple sessions.
https://support.managed.com/kb/a1816/how-to-enable-disable-multiple-rdp-sessions-in-windows-2012.aspx