I have 2 Phaser 4600 with the latest drive 5.37736.0N 2014.06.11 and firmware and I am able to select 2 sided print only on one of them.
They have exactly the same configuration.
Hi lliwaway,Thank you for using the Xerox forum.
Since the duplex printing is an option, maybe something became loose and the machine doesn't see the assembly installed.
I would suggest that you review the following solutions and re-seat all of the effected plugs to ensure good connectivity.Solutions for : How To Install the Duplex Unit
I looked at the Xerox 4600 support page and searched for Duplex as you can see below:
If that does not resolve your issue you may consider contacting our specific product support experts at the following link:Fast Track to the Expert