Hi,
You are talking about Copy function here (similar to photocopy machines - many people still call Xerox). The ADF on the Officejet Pro 8600 N911a does not have the ability to make a two-sided copy. This is because the ADF does not have a duplex unit to pull the original document back in and flip it so that the other side can be copied. The print function has duplexer unit.
You can make two-sided copies from the flatbed by setting the option and flipping the page after the first side is copied.
Regards.