"Merging files with different resolutions - I don't think that would be a great idea. Another scaling filter problems might appear."
no need to scale.using blender nodes exr files can be added,offset ,cropped etc(like in image editor).so add a image of say 640x480 and 650x480 and crop the unwanted.this will work if the two renders differ in illumination(mlt seed).i don't know the technical side,but this workaround was suggested for kerkythea.(which lacks merge function).
born to fly