Suppose you want to use the laptop named Ext as the extended monitor of the main laptop named Main.

On Ext:
Go to Settings. Search for "Projecting to this PC". Make sure the switch under "This PC can be discovered for ..." shows the word "Off" (which really means "On").

On Main:
Settings -> Display. Select "Multiple displays". Click "Connect" button to the right of "Connect to a wireless display". Click the name "Ext" (which is its computer name).

On Ext:
Click OK to accept.

If Ext does not behave like the extended monitor, select "Extend" on Main.

To disconnect:
On Ext, go to Settings -> Display. Click "Extend these displays" button. Select "Show only on 1" -> "Revert".