Use this page (Configuration > Devices > System Settings > Add) to add a FastEthernet interface IP address. The second interface is an optional feature, and can be used as a redundant interface (for fault tolerance) or as an active interface (supporting a GRE Tunnel configuration, for example).
For a redundant N+1 configuration, this option is greyed out, as the interface takes the static IP address of the the primary interface. For an active configuration, the second Ethernet interface must be configured with a static IP address to a different L2 domain as the primary interface.
The static IP address must be set for an active port assignment, and is unavailabe for a redundant assignment.