If you are printing a combined document (which consists of 2+ files) in double sided format, you will need to choose a Production Instruction option on the configure page.
Under 'Multiple files', there are two options:
Print as continuous document
Start each file on a new page
If you need to keep each file separate with regards to double sided printing, choose 'start each file on a new page'. If relevant, a blank page will then be added to ensure each file starts on a new page.
However if you are happy for one file to start on the back page of the previous file (if the page count is an odd number), choose the default setting of 'print as continuous document'.