Not sure right now how output settings are stored by KWin, I'm sure I'll learn about it over time.
But even in general, brightness storage can use a non-unique ID and that should be okay even for different monitors of the same type with the same EDID. Loading a monitor's geometry incorrectly for two of the same monitors next to each other is bad. Loading the same value to initialize both of their brightness does not seem problematic.
Anyway, this is out of scope for the MR here, more to come at a later point probably.