You can use a portrait reindition and have two facing pages displayed when you display the book in landscape, there's an option "Facing Pages in Landscape" that is only visible when you use a portrait rendition.
Or, you can of course use landscape rendition and split the layout in two, manually
Thanks - that does confirm what I was doing previously.
For some reason this book is not displaying in two page layout.
Rendition - Portrait
Export HTML file - Appearance and Behaviour: Facing pages in landscape
The original page size was 850 wide and it is an imported PDF.
Using Windows version.
I wonder if this is a new issue or something directly related to PDF imports?