https://superuser.com/questions/472346/remote-desktop-use-two-out-of-four-monitors
Microsoft looks to have at least partially implemented this feature in the latest updates to Windows 10 MSTSC.exe using a .rdp connection file for a given server.
The key settings are
use multimon:i:1 selectedmonitors:s:0,1
As of now, selectedmonitors must be set from this file, but use multimon can also be set from command line or GUI.
To get a list of target monitors, you can use
mstsc.exe /l