I've installed the X-Plane 11 demo for testing in my simulator room. I've got three projectors connected to a GeForce GTX 1070 graphics card using one HDMI output and two DP output connected to DP->HDMI converters. My problem is that X-Plane 11 start up as fullscreen on the middle (main) projector and there is no way I can make it start in windowed mode. In Settings > Graphics > Monitor Configuration there is no option for "Windowed Simulator" on any of the three monitors displayed on the bottom of the screen.
So, why do I not have a "Windowed Simulator" option in X-Plane 11 Settings > Graphics > Monitor Configuration?
To complete this question I must mention that my X-Plane 10 (side-by-side) installation successfully runs in windowed mode using the same setup.