Unfortunately, we cannot accept front and back artwork on one single PDF page, due to our imposition processes. Instead, please provide a 2-page PDF, with the front of your job on page 1 and the back of your job on page 2. Alternatively you can supply 2 separate files, one for the front and one for the back. Ensure these are clearly named so we can distinguish the front from the back.
Read this article for artwork guidelines on multiple sets orders.