Thanks for the reply. Unfortunately, that option selector in the menu that I found is grayed-out and therefore not changeable.
The menus of the latest 6180DN GPD PS drivers for Win8 do not exactly match the layout shown in the screencap of your 6180MFP PCL6 driver, and they have no plain "Options" tab. However, there are three similar sections in the GPD drivers that I have installed, as shown below:
This Printer Properties, General tab shows "Double-sided: Yes" in the Features.
But then the Properties, Configuration tab shows "Duplex Module: Not Installed", and that setting is not editable.
And, in the Printing Preferences panel we see that "2-Sided Printing" is grayed-out and not selectable.
Any Windows 8 print job that I try to send to this printer will not allow the 2-sided option to be selected.
Note that these GPD drivers do acknowledge that this is a Phaser 6180DN model, which includes the duplex feature, and in one of the three control panels here, it did show that Double-Sided function was available. Also to note is that the Windows 7 computers with the Windows 7 Postscript driver have no trouble printing 2-sided pages on this printer, and all of their control panels show that the duplex module is installed and functioning.
So, it still looks like the problem is caused by a bug in the Windows 8 printer driver software, for which the developers should be notified.