Follow this link: http://woshub.com/how-to-allow-multiple-rdp-sessions-in-windows-10/
It happens that when you try to establish a second RDP connection under a different user account, you see a warning:
The number of connections to this computer is limited and all connections are in use right now. Try connecting later or contact your system administrator.
In this case, you can use the Local Group Policy Editor (gpedit.msc) to enable the policy “Limit number of connections” under Computer Configuration -> Administrative Templates -> Windows Components -> Remote Desktop Services -> Remote Desktop Session Host -> Connections section. Change its value to 999999.
Restart your computer to apply new policy settings.