On the printers IP interface make a shortcut for a network share. In the set up you will see an option that says "Sides" with choices of "short edge" and "Long edge". This is duplex scanning, and while I have B315s, I assume the C315 probably is set up similarly. Took me forever to figure this out because the wording isn't very clear so I didnt realize thats actually the duplex scanning setting.
Even Xerox didnt know this when I asked how on the phone or in these forums lol