Chapter 9 High Availability for FortiOS 5.0 : HA and failover protection : Remote link failover : Monitoring multiple IP addresses from one interface
  
Monitoring multiple IP addresses from one interface
You can add multiple IP addresses to a single ping server to use HA remote IP monitoring to monitor more than one IP address from a single interface. If you add multiple IP addresses, the ping will be sent to all of the addresses at the same time. The ping server only fails when no responses are received from any of the addresses.
config router gwdetect
edit port2
set server 192.168.20.20 192.168.20.30 172.20.12.10
end