If the adjustments in A are independent of the adjustments in B, then you copy the settings from A into C, then copy the settings from B into C.
If they are not independent (for example A has exposure –0.5 and B has exposure +0.5), then the question seems meaningless to me.
It's not a global setting, it's local adjustments with the brush that I want to copy
They seem to be replaced..
Off the top of my head, I don't think you can merge these from two different photos to a third photo.
Maybe if you were really smart and wrote the edits to XMP and then edited the XMP with some sort of text editor, you might be able to merge the results, but that's pure speculation on my part.
I will give it a try manually