I found that this happens when there is an object (in may case it's a library object that I drag onto the page that gets flung into the white area off the page and gets pasted off the spread. Creates a huge white space between pages.
Zoom out so you can see both pages and select all and see if you can see a bounding box for the object between the pages where the white space is and click it and then remove it.
This should then remove the offending white space between your pages.
Hope you find this helpful.