Do you have access to InDesign?
Yes. i have CS5
I just did this for a client this week.
Start by doing your 1-up version and use Print Booklet to make a PDF (what version of OS X? You may need to install the adpdf9.ppd and print to postscript, then distill -- see Print booklet to post script does not work | Adobe Community ), or use one of the imposition scripts floating around. In any case, you need an imposed PDF in spreads of one copy.
Now get Scott Zanelli's script from InDesignSecrets » Blog Archive » Zanelli Releases MultiPageImporter for Importing both PDF and INDD Files and use it to place your PDF twice into a new 8.5 x 11 document. I rotate one copy 180 while doing the import so it is foot-to-foot on the page to allow a little tighter margins at the bottom, but you may not need this.