Merge these two images in a way that retains their highest and lowest brightness valu

[Redrobes]

Merging the brightest and darkest parts is not too hard but the arbitrary gradient in the middle would be a bit more difficult unless you can apply a math curve to the pixel brightness values - which you can do but paint apps is not going to make that easy.

But to get the darkest parts of the bottom image onto the top image you take the bottom image and use the curve function on the bottom image to cut off all pixels brighter than a certain amount then take top image and cut off all pixels darker than certain amount and then merge the two using brightest of each.

But as you are rightly saying, whilst the pixel brightness will be continuous, there will be a bit of a discontinuity in the gradient of pixels. So the images I will put below will look fine but when viewed with a height map viewer will not look quite so good.

I'll edit in some pictures in a moment...

So this is Top High, Bottom Low, Top Mid, Bottom Mid and then All which is the combined version of all of these temporary images. The result is not ideal I would admit...