Using spreads
Spreads are pages which spread across two pages - hence the name. So if your book uses normal pages measuring 300x400 a spread would be 600x400.
Since the software is written to manage single pages you have to enter a spread page twice and tell the software it is a spread like this :-
<content ... >
<page src="pages/FrontCover.png"/>
<page src="pages/page1.png" isSpread="true" />
<page src="pages/page1.png" isSpread="true" />
<page src="pages/RearCover.png"/>
</content>