We do not advise doing this, it can be easy to overwrite your project, and this cannot be retrieved. It might be necessary though in case your book turns out to be bigger than the 100 pages maximum per book.
Here is a pretty safe way to split a project into two separate projects:
When you open a saved project, you are asked to either choose “edit” or “Apply this design to a different project”. Then you can save the existing project under a new name and can then delete the pages that are already in the first project. Do the opposite with the first project. Make sure you save your projects frequently and in every case before switching to the other project!
Tip: Have a back up copy of your project on a CD or SD card, just in case you overwrite your original project.