I have a book with multiple documents. Document A ends on a left-hand page, document B starts on a right hand page, but when I export the book as PDF SPREADS, the last page of A won't join with the first page of B in the spreads PDF. I've tried dozens of variations on page numbering, layout, etc. No use.
ID cannot create spreads with pages from different documents.
If you want spreads on a pdf (I'm assuming for viewing purposes) export the book as pages and in Acrobat: Properties>Initial View>Page layout>Two up (Cover Page) and save it.