Enable Multiple RDP Sessions
Log into the server using Remote Desktop. Open the start screen (press the Windows key) and type gpedit.msc and open it Go to Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Connections. Set Restrict Remote Desktop Services user to a single Remote Desktop Services session to Disabled. Double click Limit number of connections and set the RD Maximum Connections allowed to 999999.
But I fear how all your application supports multiple session. Terminal server or VDI is a complete solution which need to be additional cost.