How to add left parent page to a "right" single parent page?

You can set your document up in spreads using single pages (rather than facing) and apply different parent pages to the left and right document pages. You can also set up the parent pages for the document as spreads. To allow single pages to be made into spreads you must first make sure that Allow Document Pages to Shuffle is unchecked in the Pages Panel pull-down menu. Then you can pull the single pages into spreads as seen in the first picture below. It is a manual process, however. To make the parent pages in a single page document into spreads enter the number of pages you want in the appropriate area of the New Parent dialog as seen in the second picture. Once you have the parent pages as spreads you can pull either the left parent or right parent down onto the appropriate document pages. You cannot pull the parent pages down as a spread, however. If you try that only the left parent will get applied.