Here's the answer I received at ApertureExpert.com
"If I understand correctly, you basically want to create a double-page spread, meaning a single photo spans two pieces of paper (like a two-page spread in a magazine)but in a single JPEG?
The easiest way I can think of is to create a custom book in Aperture, make a single page layout that does what you want, and then add photos to the new "book". Once created, that custom template will be there forevermore for you to use. Then you can "print" the book, but choose to export as a JPEG (from the PDF menu).
Here's the basic steps:
1. File > New > Book
2. Book Type > Custom
3. New Theme
4. Customize page size to match what you want
i.e. 22" x 8.5" standard printer paper horizontal, double-wide
5. Name your theme so you can find it later, and Choose Theme
6. There should be a page layout exactly as you want already; a "full bleed". Don't choose the 2 page bleed, as that will make your page 44" wide. If you want to edit that page (i.e. to add a title, or a border, whatever), select the page, switch to Edit Layout mode, and modify as you like.
7. Delete all the excess pages. You can't delete the very first page (the cover) AFAIK, so you'll need to keep in mind that that's a throw-away page in the future, however you can delete the photo box from it in the Edit Layout mode, so you can't accidentally add a photo to it.
8. Duplicate that page to make as many of these pages as you need.
You now have a "book" that gives you the layout you want. Instead of having to go through this every time, leave that book empty and call it something clever like "master 2-page spread". Then whenever you want to output some 2 page spreads, just select that "master" book and choose Duplicate Book.
Now to actually output these
1. Add photos to your duplicate book (drag them to the book icon)
2. Add the photos to the pages. You can do this automatically with the Autoflow Unplaced Images command.
3. Open the Print menu, and from the PDF drop-down, there's an option to save "PDF to folder as JPEG". Choose that, and off you go. This is actually an applescript, so watch for the spinning gear menu in the menu bar to tell you it's doing something. Once it's generated the files, a dialog will pop up asking where to put them.
I hope that answers your question. Long answer, but easy to set up, and once set up, even easier to implement!
cheers
-Joseph @ApertureExpert"