[senseabove]

For any other Mac users out there, there's a tedious way to do this with preview.

Make two copies of the original PDF.
Open one copy, select the left hand pages with the Box tool.
Make sure the "Thumbnail" view is selected in the preview bar on the right side. Select all pages in the preview bar.
You should now be able to see where the selection you want to crop falls on each page. Make sure nothing will get chopped off, then select Crop from the Tools menu, or hit Cmd-K.
Now, you'll need to "Print" the file to a new PDF. When you crop something in Preview, it doesn't actually delete the data which you cut off, just hides it (To see the cropped document in its original, uncropped form, go to "View/PDF Display/Media Box"). However, if you print it, only the area you kept will be printed.
Repeat with the right hand pages.

And here's the tedious part.
You should now have two files, one with only left hand pages and one with only right hand pages.
If you've got both windows open in Preview, with thumbnail view in the right hand preview bar, you can drag pages from the right-hand-pages file into the left-hand-pages file, interpolating all the pages. Helpful tip: It's easiest if you go to the Edit menu and insert enough blank pages so that the page numbers on the actual document match up with the page number in the PDF file. That way you can tell if you get off number or drop a page in the wrong spot.

Anyone know of a faster way to do this? I don't have any experience with Automator, but could it automate the interpolation process?