Need fast help

[Ofnuts]

If I understand correctly:

• Open first picture in Gimp
  
• File>Open as>Layers to open the second picture has a new layer
  
• Scale/crop/move the two pictures so that they overlap as you wish. Make sur the top layer is the active layer when you are finished.
  
• On the top picture: Layer>Transparency>Add alpha channel (if it's grayed out, it means it's already there, so no worry)
  
• With the Freehand select tool (:freehand click in three corners of the image to make a triangle selection (don't forget to strike [enter] to commit the selection if you use Gimp 2.10. Setting View>Snap to canvas edges (Alt-Ctrl-E) will help you click exactly on the right spots
  
• Edit>Clear or [Delete] to erase the selected triangle in the top picture.
  

   

Attached XCF (this is a "compressed" XCF that Gimp will read just fine)(scaled down to keep it under the size limit)