Replace phases of image FFT
What you want to do is impossible with your technique. The phase of an FFT does not provide the details of an image. Even if it did, why would it enhance an image, if you exchanged the "details" of both images without any calculation?

The phase of an FFT of an image mainly holds the structure information, i.e. how the frequencies contribute to the image (in terms of phase offsets). This is valid for fine and coarse structures in the image. The magnitude tells you, how much each frequency contributes to the image. This information is in most cases less important, as can be seen by the general shape of the 2D magnitude spectrum (many strong low frequency contributions and a rapid decay towards higher frequencies).

I assume, what you would like do do is more a quite complex problem to be solved in image space. You might want to look for publications and algorithms on super-resolution for your application.