Wow cool stuff. I think some of the shots from the link can be done in camera. But it's a discipline that can take years to master. I think there's masking involved. Masking is leaving parts of the film unexposed for the next exposure or protecting areas already shot while doing an exposure. Another method is done with multiple negatives with masking of photographic paper instead of film. I love the work of Jerry Uelsman. He works with multiple enlargers and multiple negs using masks also.
His wife Maggie Taylor is also talented, but she uses a computer.

Chances are, it's not a double exposure, but multiple exposures. It takes previsualization and planning the shots. You have to think out the shots thoroughly. Good luck!