The pdftk tool has this multibackground feature for merging (i.e. not concatenating) 2 PDF files.

Besides adding a real background to a PDF this is also useful for adding a text layer to a PDF (where the text layer PDF comes from an OCR program like tesseract).

Example:

pdftk images.pdf multibackground ocr-textlayer.pdf output document.pdf

Thus, it would be great if stapler would add such an operation, as well.