You could try setting the backglass height and width to 1x1 so it won't really be visible
Now that there is an extra monitor, topper, supported, you might be able to do what you are trying to do. Set Backglass monitor to you #2 monitor in your diagram and the topper monitor as #3 in your diagram. Use pbxrecorder to record 2 videos each with 1/2 of the backglass being recorded