I have the same setup - two external monitors plus my Mac Air built-in display.
I'd like to be able to select only the two external monitors for the RD session while leaving the built-in display for other stuff.
On Windows it works by manually editing the .rdp file and setting the selectedmonitors property to only a subset of monitors. It would be great to have this supported by the Mac version as well but it seems it's not supported.
One workaround I was able to do but it's not ideal:
- In Mission Control make sure the option Displays Have Separate Spaces is Enabled.
- Configure your RD session to use All Monitors
- Start you RD session. This will create RD session stretched across all screens.
- Move all your windows inside RD session only to the two screens you wish to use, leaving the screen on e.g. the built-in display empty
Activate the Mission Control by pressing CTRL-Up
On the built-in display select the other desktop you want to display there
Now you have your Mac Desktop on the built-in screen while the external monitors display the RD session
The caveat is that if a new window inside the RD session opens up on the third screen you won't see it, but you can always invoke the Mission Control, switch the Desktops to display the third RD screen, move the window to the other two displays and then switch the Desktop back to Mac.
So it's a workaround, not a solution. It would be great if the RD client supported selecting just a subset of monitors to use.