The page numbers are repeating objects and a double page spread is a second spread on the same page. Repeating objects, when set to repeat on all pages are automatically applied to a new page and not to a new spread.
To apply page numbers to the spread of a page:
- Copy the repeating page number object from the first page. You may want to include any borders or decal as well that are repeating
- Paste the copied page number on the new spread and align it as required. The number should of incremented to 2.
- Highlight the page number and any other objects you have copied to the new spread, then right-click > Repeat on all pages
Now any new spreads that you create will automatically have the repeating number at the bottom.