Nawras Pages Merge is a freeware that allow you to join two PDFs into a single document by alternating the pages.
Download link: Nawras Pages Merge
More and more multifunctions printers have a tray for automatic photocopying and scanning which makes these functions easier.
Unfortunately, some devices scan only one-side of the paper. Therefore, you need to run the scanning process twice, for each side of the paper, without the possibility to combine correctly the output PDFs.
Nawras Pages Merge was created in order to combine the two PDFs (for odd and even pages) by alternating the pages.