Dan, you can use the free Montax app (see link in earlier post) to do what you want to do:
  • Export the Xara file as a .pdf with the page size you require (quarter Letter)
  • Import the .pdf into Montax
  • Apply the settings as shown in the screenshot below. (I used the A4 setting but you can select 'Letter' in the box with the arrow pointing to it. As you can see, I imposed 24 A6 pages.)
  • Export by clicking File/Export imposition into PDF
  • Print this file double-sided using the appropriate printer settings (not using the booklet setting)
  • Cut the printed sheets in half
  • Rearrange the cut sheets to form booklet.

Click image for larger version. 

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