PDFsam works for merging scans of double sided documents

Occasionally I need to scan a double sided document. For a short documents (1-4) I just scan the fronts of the pages and then the backs and then manually put them together using Preview in Mac OS X.  However for longer documents the free PDF Split and Merge, or PDFsam, quickly merges these types of scans:

http://www.pdfsam.org/