Go to a page that is not the first or the last page of the book, since they can only be viewed in single-page view, and click on either the single or the two-page buttons in the lower left corner of the screen.
Please note that you cannot view a linked two-page spread in single-page view.
Also, changing the view to two-page view only allows you to view two pages at a time. It does not link the pages together as a two-page spread, which would allow items to be placed across the gutter.
To place photos (or other items) across the gutter, see “How do I place a photo across two pages?”