Here is the sample of bonding two interfaces for link aggregation.

#interface configuration
interface name "port1" "outside"
interface name "port2" "inside"
interface speed "port1" 1000full
interface speed "port2" 1000full
 
#bond configuration
bond interface "bond1" "inside"
bond interface "bond1" "outside"
ip address "bond1" 172.16.0.253 255.255.0.0

ip address <bond_ifname> <IP_address> <netmask>
Where,
bond_ifname is a network interface name specified by an alphanumeric string; its default values are bond1, bond2, and bond3.
IP_address is the designated IP address of the interface (in your case, you want to use the outside IP address)
netmask is the appropriate designated netmask.

In Webui Array OS 6.4 or later,
Under "System Configuration" -> Basic Networking -> "Interface" tab -> Link Aggregation ->"Add Bond"