By default RDWeb in Windows Server 2012 R2 only displays published RemoteApp programs. One can also display a full RDP desktop session by changing the registry value ShowInPortal to a value of 1:
The full remote desktop session will now be published to the session collection. However, even if you have set relevant GPO on your session hosts to not allow multi-monitor you will find that it still spans two displays, which is particularly annoying if you need to view content on both your local machine and the remote machine.
To disable this edit the RDPFileContents key which can be found in the same node as the ShowInPortal key which you already edited. The REG_SZ key is very long but a little way in you will find the following:
amend this to
and then you will no longer get sessions spanning multiple screens. Note that much like the ShowInPortal key if you make amendments to your session collection via the GUI these keys can get reset back to their default. You may wish to use DSC or a GPP to ensure they remain set how you like them.