In Blender, simply mix two image layers like in Gimp In Gimp I've got two layers with identically images with text in middle grey in the foreground on a black background. The lower layers's mode is set to 'normal' while the higher one's is set to 'addition'. 

Moving one layer against the other leads to the effect, that those image parts alter to light grey where the middle greys overlap.



How can I achieve this behaviour in Blender?
Edit: Ok it looks like it's not what you're looking for, you want 2 different planes and I give a solution for one plane:

You could simply mix your 2 pictures in a Color > MixRGB node (set on Mix):