Dave, that has to take the prize for Layout Tip of the Year! I had no idea this could be done (I’m on SU 2018 so presume it works in earlier versions as well?). I have a two screen set-up so had a go at getting nearer to the OP’s original intention.
Open a Layout file
Click on the Maximise box icon at the top right corner of the window to get the overlapped boxes icon
Click and stretch the window edge across both screens and adjust the upper and lower edges if needed.
Open a second Layout file and right click on it’s tab etc as Dave has shown
Drag the dividing file window edge across to the screen edge between the monitors
Result is two full sized windows of two Layout files over two screens
I am running Windows 10 and can’t say if this will work for everyone but give it a try!
Incidentally, you can have more than two tiled windows but it does get a bit crowded.