Chapter 9 High Availability for FortiOS 5.0 : HA and failover protection : Link failover (port monitoring or interface monitoring) : Example link failover scenarios : Example: port2 on FGT_1 and port1 on FGT_2 fail
  
Example: port2 on FGT_1 and port1 on FGT_2 fail
If port2 on FGT_1 and port1 on FGT_2 fail, then FGT_1 becomes the primary unit. After both of these link failures, both cluster units have the same monitor priority. So the cluster unit with the highest device priority (FGT_1) becomes the primary unit.
Only traffic between the Internet (port1) and DMZ (port3) networks can pass through the cluster and the traffic is handled by the primary unit only. No load balancing will occur if the cluster is operating in active-active mode.