Pi 4, 2 monitors with same display - screen layout changes not being respected

Question

I have 2 HDMI touchscreens that I'd like to have with identical vertical displays. By identical, I mean I'd like the screens to be mirrored and show exactly the same image.

The Screen Layout Editor is not working for this. I am able to rotate the screens. I am able to drag them around as long as they don't overlap. I am asked to reboot each time I make a change.

As soon as I stack them on top of each other in the layout editor and reboot, they default back to the default horizontal side-by-side configuration. What am I missing to make them display the same thing?

I have an older Pi 4 that I've done this successfully with so I know its do-able. It is also what is recommended all over the internets. Any help is appreciated.

Lastly, I attempted to install a new OS and was able to reproduce this problem on a fresh OS install.

Edit: I dug up an old image that I have that works with same hardware. Working one is running buster and the one that is not working is on bullseye. Can someone please point me to some config that I should look at that may be different? Looking at /boot/config.txt the only difference seems to be dtoverlay=vc4-kms-v3d vs dtoverlay=vc4-fkms-v3d(working), but changing this property does not fix the issues.

Answer 1

I have run into a similar issue with Bullseye, but it doesn't even ask me to reboot. It does apply the changes I make, but any time I reboot, it does not remember the settings and I have to reconfigure it every time.

I worked around this issue by using xrandr.

1. I built the xrandr command that would set the screens the way I wanted (ref: https://x.org/releases/X11R7.5/doc/man/man1/xrandr.1.html ; archived: https://archive.ph/JNVsl)

xrandr --output HDMI-1 --primary --mode 1024x768 --rate 60.00 --output HDMI-2 --mode 1024x768 --rate 60.00 --right-of HDMI-1

2. I added a script to the GUI start up script area (ref: https://www.makeuseof.com/how-to-run-a-raspberry-pi-program-script-at-startup/ ; archived: https://archive.ph/qskAf)

sudo nano /etc/xdg/autostart/display.desktop

Script:

[Desktop Entry]
Name=SetupDisplays
Exec=xrandr --output HDMI-1 --primary --mode 1024x768 --rate 60.00 --output HDMI-2 --mode 1024x768 --rate 60.00 --right-of HDMI-1

Now when I reboot, after the GUI loads, it runs my script and puts the monitors in the configuration I want.