How to configuring two monitors to overlap on each other

I have a laptop monitor and an external monitor that I have configured in kde to overlap but when the laptop is rebooted I would like to be able to have fvwm2 (in this case) to continue to overlap the monitors by default. I am having to log in as root with kde, tell the system to overlay the monitors, and the logout. When I then login to fvwm as a normal user the two screen overlap each other as if one screen. What might be the appropriate configuration in the .fvwm2rc file to have fvwm overlap the two monitors automatically instead of having to have kde do the initial setup?

You can use a graphical tool like arandr to configure your monitors to overlap as you wish. Arandr can create a sh file with the config. Then you can execute the from your fvwm config.

Alternatively you can do the same by using purely xrandr and bypassing graphical tools (check the man pages or --help of xrandr for details).

For instance, I do use xrandr to lower the gamma by having this line in my StartFunction:

+ I Test (Init) Exec xrandr --output eDP --gamma 0.8:0.8:0.8 --set TearFree on --primary

Thank you for the information.