Whether you want a single full-screen RDP connection or a Multi-Monitor full screen RDP connection it's a very simple edit.
Please note depending on your environment the RDP icon may be deployed through Group Policy in which case your edits will revert back upon the next login to your workstation.
Close your current RDP connection by hitting the 'X' on the blue toolbar that appears at the top of the RDP connection (You may have to move your mouse to the top of the screen to get it to display if it is not pinned).
- Right click the RDP icon that you use to connect to your remote.
- Click on edit.
- On the dialogue box that pops up go ahead and hit the display tab. (You may need to hit show options to see the different tabs.)
- Use the option highlighted to toggle whether you want to use both monitors or only one for the RDP connection.
- Go back to the General tab and hit save at the bottom.