Hello, my name is Syd and I am with Epson America.
For double sided printing with this size, scanning to a computer would be required.
If the source is 12 single sheets of paper that is only printed on one side, you can use the ADF and insert all pages at once. Use Epson scan to save to a PDF file on your computer.
If the source is from a book, the book can be placed directly on the glass and run Epson scan from a computer. As before choose PDF, and Epson scan will ask if you want to save now or add another page to the file. Then place the next page of book on the glass and click continue. This will need to be done for each page you want scanned.
Once the PDF file has been created, you can simply print this file out choosing the manual duplex option. The printer will first print all the odd pages, and then ask you to turn the printed paper over and re-insert. Clicking continue, the even number pages should be printed.
You may want to contact tech support at 562-276-4382 or practice with plain letter size paper as there are multiple steps involved with this size.