- Just use one Blender file. That will keep things simple for editing your combined shell. Choose one shell file as your base, then absorb the other shell file's geometry into it and work from there.
- Put all objects on the same page. Again, for simplicity's sake. This just requires changing the page_num logical property to match the initial shell number.
- Declare the other shell(s) on the Book text object. When you export your combined shell, it will then create a PRP for each shell, even though only one will contain the combined objects. That way, when you submit your PRP files, they will overwrite the older version of the uncombined shell(s).
These are just suggestions. Feel free to add to or debate them based on your experiences, if you like.