Hi, and thanks. A merged slider could be interesting. My main question would be how to figure out how to translate the minimum & maximum brightness values to the respective screen. Would a percentage the unified slider mean the same percentage on all screens, regardless of how bright they are in practice? Or would we need a config UI to determine the brightness range for different monitors independently?
Either way, this would be a different issue. Please open a bug report (with "wishlist" priority) in the Brightness and Color applet category for plasmashell. Also note that pressing the brightness key on a keyboard or scrolling the applet icon will adjust the brightness of all monitors in Plasma 6.2.
I also noticed a small issue, if you set the brightness to something on the "Brightness and color" widget on one monitor, the changed value is not reflected onto the other monitor.
Plasma 6.2.4 (which was released today) has a fix for this :)