The remote desktop connection app has a setting "Use all my monitors for the remote session". But, what if you want the remote session to span only some of your monitors - and not all. To do that you need to edit the .rdp file (in notepad) and add the following like to the configuration.
selectedmonitors:s:0,1 You can get a list of all your monitors by running this command. mstsc /l
I edited the Default.rdp file, so this works for all my remote desktop sessions, but you can edit a specifc .rdp file - just make sure to start the remote session by doublicking on that particular .rdp file.
You can find the list of all RDP display parameters here: Supported Remote Desktop RDP file settings | Microsoft Docs