Even if a service looks similar to another, doesn't actually mean you can migrate the content created on a website to a different website. However, for photo books, try to export the project and not the final product. The project should have the same extension name on both apps from both websites.
After this, try importing the project back into the other app to see if it works. If it doesn't work, then compatibility is missing and you can't use the same project on two apps from different websites.