Quite often I receive a document to print double sided but I receive it as two single PDF files. Other times I have two documents that came from two different applications (for example one from pages and one from numbers) and I want to print them back-to-back on one piece of paper using my double sided printer. Here’s how to print to PDF files together using OS X Preview.
Make sure both the files are PDF files. For example if you have a Pages file you can go to the ‘Share’ menu and choose Export PDF.
Open the two files in preview at the same time. Select them both in the finder, and double click on one. They will both open in the same preview window.
When both files have opened in the same preview window, you can simply choose print and print them double sided even though they are two separate files.
Of course you could achieve the same thing by merging the two pdf files, then printing them, but this keeps them as two separate files and it is quicker.