is there an easy way to keep two masks synchronized with one the inverse of the other

I have a huge pile of very old slides.  The colours are very wonky and often I need to use adjustments on selections.  The most common case is two layers, one called Sky and the other NotSky, with suitable masks.  Gradients work sometimes, but rarely.  But horizons often have trees and other irregular features and the selections are not easy to define, particularly when one needs to be the inverse of the other, and tiny errors create bright or dark lines at the boundary.  The obvious strategy is to load selection from one, then use select and mask with inverse to make the other.  This works if I get everything right first time, but when I don't, I end up chasing my tail trying to keep the two masks well synchronized.  Is there something easier I should be doing.