With pdf-tools you don't need an expensive duplex scanner! All you need to do is scan the odd pages into a single pdf, flip the pages over and then scan the even pages into a single pdf (the order is backwards but read on). Then with pdf-tools (split/merge), reverse the order of the pdf with the even pages, split the odd page pdf to separate pages using a, split the even pages to separate pages using b and then merge these - in alphabetical order - into a single document. I love it. It takes me all of 45 seconds. Fantastic.