Can double-exposed film photos be separated digitally?

It's possible to work at this slowly through an iterative process. It takes a lot of effort and will take some artistic skill to get good results.

The basic process is:

1. Make a copy of the layer in photo editor.
2. Use clone brush and smudge to remove all details that are not part of the image in this orientation. Fill in with the color you expect should be there. Don't worry too much about removing detail, because it will come back later.
3. Set the mode of edited layer to Subtract (note: not the same as Difference).
4. You should now get most of the other photo. Adjust Brightness/Contrast or Levels setting on the edited layer to get the best result you can at this point.
5. Copy Merged to new image, rotate by 90 degrees and work on the opposite image.
6. Repeat steps, always subtracting from the original and then working on the opposite image. Note that subtraction should always be between the original image and the edited image, never between two edited images.

Any detail you remove from one image will appear in the other one. You'll need to figure out somehow what detail belongs in which image.

Here is what I got with this method in 4 iterations. I admit it is not great, but might act as a starting point for sufficiently skilled artist to improve.