Step 1: Install TightVNC application


Step 2: In the server settings on the "Server" tab in "Update Handling" square set checkbox "Use mirror driver if avaliable"
As a result, when connected, we get a canvas of several screens.

Step 3:  Go to the "Extra Ports" tab, 
You can read description  in the bottom part of the window (Help).

For two FullHD monitors it turns out like this:
5901: 1920x1080 + 0 + 0
5902: 3840x1080+1920+0

Where:
1920x1080 - total screen size
1920+0 - Point from which crop total screen size to the end

In summary: when connected to a standard port (5900), we see the canvas from all monitors,
when connected to 5901 - see the 1st monitor
when connected to 5902 - see the 2nd monitor