Comment 2 for bug 1905153

Laptop brightness is unfortunately not an attribute of the screen itself, but of the backlight. So Linux can only easily control the brightness of those screens with known backlight interfaces, which means the laptop monitor only.

If you wish to adjust the brightness of any other monitor then you should do so using the monitor's own buttons so as to not lose colour detail. If you don't mind losing colour detail then you can also use something like:

  xrandr --output NAME --brightness VALUE

Even if interfaces did exist to implement what you want, we could still never know the relative brightness of different models of LCD panel. So choosing one brightness value for two different screens would be kind of useless. Safe to say this will never be implemented, at least not as a single brightness value.