hey there, this makes perfect sense! so now i hope i make sense 😅 basically, you can't have 2 versions of a .world file installed. if we use moonwood mill as an example because it only has a single .world file, you could only have the sunblind version OR the milk thistle version of that world installed. but, if we use a world like tartosa which has 2 neighborhoods which means 2 .world files, you could have the milk thistle version of one file and the sunblind version of the other file installed. similarly, if you like how a lighting mod looks in one world but prefer a different mod for another, you can pick and choose which worlds you install different mods in. hope that's clear!
now, for the preset you're talking about: milk thistle and sunblind are actually pretty similar in their general lighting. most of the differences between the 2 lies in sunrise/sunset lighting and horizons, but you should be able to just pick which lighting mod you prefer and use the preset with little to no additional adjustments needed.