Auto adjust colors so that second image colors matches

[Ofnuts]

What I tried:

• Pick up a "raw" picture in my collection, and load in RawTherapee
  
• Output two pictures as JPEG, one with no exposure correction, and one slightly more exposed
  
• Load these in Gimp as layers
  
• Set the top on to "Difference" mode
  
• Start Colors > Exposure and try to make the difference as dark as possible.
  

Doesn't work too well: I can't make the whole picture go dark, some areas brighten before other are finished darkening. Tried with Curves and I get better results (darker difference), but that requires some tweaking. I suspect that the exposure fix is distorted by the gamma correction.

Also tried to use Colors > Auto > Equalize on both pictures. This works fine from the point of view of having both image look the same, but they both look awful...

What I would try if you take the pictures "raw":

• Looking at the EXIF you can tell exactly the exposure value (EV) of each pic, and therefore compute the difference. 
  
• Your demosaicing app very likely has an exposure compensation slider with EV values, so you can correct one of the pictures
  
• Possibly some auto-exposure on both images could give better results in that app than in Gimp.