To toggle between single and multi monitor mode, right click on the rdp file and select edit
From here you will need to select the display tab
On this page there will be a checkbox where you can select to use all of your monitors or just one.
To do this, follow the steps above to set the checkbox Use all my monitors for the remote session to `checked`
Use all my monitors for the remote session
You will need to take note of what monitor is numbered what. run mstsc -l in command prompt to see what monitor is what.
mstsc -l
Right click on the RDP file and open it with notepad.
Once in notepad, find the variable selectedmonitors:s. It will have the monitors listed that you wish to use. In my case I set it to selectedmonitors:s:1,2. This may take some trial and error with the numbers.
selectedmonitors:s
selectedmonitors:s:1,2