Scott Hanselman has put together a quick and easy writeup on how to use remote desktop with just a subset of available monitors here. Should the link ever go down, I wanted to document how I use this myself, borrowing from Scott's description.
The "Use all my monitors for the remote session" in Remote Desktop is useful, but selects all monitors. Sometimes, there's value in keeping one or more monitors linked to the local PC. For example, since remote desktop makes my audio/video challenging, I tend to run my communication tools (Skype, Teams, Zoom, Slack, etc.) locally.
Fortunately, someone at Microsoft provided support for choosing a subset of monitors, even if its not UI friendly accessible in the dialog. We'll need to edit your personal RDP settings file, but its not bad, and you can always delete it to revert back to default if needed.
Step 1: Get the list of monitors
On the commandline, run:
mstsc /l
to get your list of monitors. Mine looks like this:
Step 2: Edit your Desktop.rdp file
Normally this is located in your personal Windows "Documents" as Desktop.rdp (hidden/system file), but you can save it anywhere, under any name.
If you open it up, it looks something like this:
full address:s:x.x.x.x:3389 prompt for credentials:i:1 administrative session:i:1 screen mode id:i:2 span monitors:i:1 use multimon:i:1
To get a subset of monitors working, using the IDs from step 1, add (or edit, if already present) the following lines
selectedmonitors:s:27,42
Where 27, 42 are the IDs for your monitors. eg. 1, 2 are more likely values. You can keep adding - comma separated - more monitors to this list.
You'll want to make sure you also have the following in your .rdp file:
use multimon:i:1
so RDP knows to use more than one.