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