Apologies - i meant pdf file. Unsure what happened there :Z
Perhaps two scenarios..
The page to insert will always be a single page, to be added in as the even (or back page) to the original file.
Second scenario: both files will be the same page count, eg 36 Fronts.pdf 36 backs.pdf, and they'd be combined as Odds (fronts) Evens (backs) > output.pdf
If for instance one file was a different count to the other, blank pages could be added?
The first scenario is the main application though. it basically works in the opposite to the current Insert/Append function, whereby instead of in the box "pages to include" being able to enter "odd" "even" "all" etc, the insert/append info section allows for the original file to say "after each page"