I think using the Align tool
Open the first image stack
Turn the visibility of everything off (need for the Align rubber banding)
Go to the top of the stack
Open As Layers the second image stack . That should now be above the first (invisible stack)
Make a path (for the Align tool) along the bottom edge of the image.
Open the Align Tool and rubber band, (like making a selection) the whole image.
Choose Relative to: Active path
Click on Align top edge of target.
Image -> Fit canvas to layers
Make everything visible.
Reads more complicated than it is: All in 45 seconds: https://i.imgur.com/4Tjgci2.mp4
Open the first image stack
Turn the visibility of everything off (need for the Align rubber banding)
Go to the top of the stack
Open As Layers the second image stack . That should now be above the first (invisible stack)
Make a path (for the Align tool) along the bottom edge of the image.
Open the Align Tool and rubber band, (like making a selection) the whole image.
Choose Relative to: Active path
Click on Align top edge of target.
Image -> Fit canvas to layers
Make everything visible.
Reads more complicated than it is: All in 45 seconds: https://i.imgur.com/4Tjgci2.mp4