FortiWeb Reserved Administration Interface

I have two FortiWeb 400F units in Active-Passive HA mode. The management IP addresses on port 1 are x.x.x.180 (Primary) and x.x.x.181 (Secondary). When accessing the web interface, I can only log in to the active HA unit (x.x.x.181). I understand that access to the GUI is limited to the active unit because the "Reserved Management Interface" option is not enabled; however, when attempting to configure the reserved interface on port 1, the port does not appear as an available option for assignment.

What configuration steps must I take to enable GUI access to the passive unit as well?