Hi, PDFClerk can in fact prepare PDFs for printing 2-up with non-duplex printing, and do so such that when you have assembled the paper you have a real booklet that you can fold (or cut) in the middle, which you cannot do with the (free) alternative built in to Mac OS X suggested earlier:
You first set the factory-provided 2-up imposition and then export to PDF by using the File->Export->Split Into Even/Odd Pages command from the menu bar. This creates two PDF files, one with the even pages and the other with the odd pages. Print, turn over the paper, print the other document back to front, then fold (or cut if the books are big) and you have your book/magazine in true book form with all pages in the correct order.
(Apologies for the English. I can read Swedish, but not speak it.)