Can't set Full screen non-native resolution on 2nd monitor/display

I'm working with a 3 physical screens 3d stereoscopic CAVE setup. Each CAVE screen has it's own dedicated slave render pc which is connected to 2 stereoscopic beamers . Each beamer outputs 2 separate images for stereoscopic 3d. Also, each beamer pair's output is combined into a high resolution single virtual display using Nvidia Mosaic and we use Vioso Visualblend to do blending and warping on an OS level (Windows 10).

Since we use a lot of screen estate (basically per render slave we use 2x ~4k monitors which ideally run at 120 hz/fps or 60 hz/fps per eye), I'm exploring ways to save performance. I'm experimenting with lowering the rendering resolution but I'm having trouble to lower the rendering resolution on any 2nd monitor while keeping the program full-screen.

I tried all sorts of combinations of FullScreenMode both by script and by player settings and also used both available API's in Screen class and Display class but I haven't found a way to lower the resolution on both displays.

The Screen class methods only seem to apply for the primary monitor.

With the Display class methods, I get undesirable results. For example, when I set both available displays to a very low resolution like 640x800, the second monitor doesn't display the program in full screen anymore. See screenshot below where I did a quick test on my local pc with 2 monitors attached. The right side (primary monitor) displays in full screen (although with black bars due difference in aspect ratio, but I don't mind this as the CAVE screens are in correct aspect ratio) and the left side renders the programm not in full screen.

Is there anything I could try from Unity side of things? I could change the resolution on an OS level, but this would affect our current blending and warping profile, which is not ideal either.

I tried with 2019.4.12f1 and 2020.3.13f1.