From the Apple website
Use Preview to open up the two PDFs you would like to merge. Choose View » Show Sidebar (or click the Sidebar button). Make sure both PDFs are visible on the screen at the same time. When the Sidebar pops out, you will see a graphical representation of the pages in your PDF document. Simply drag the page, or pages (use Comand to select multiple pages) from the Sidebar of one PDF to the Sidebar of another. You have now merged pages from two separate PDF documents.
So easy. So simple. So elegant.