First Step on the Master Firewall:
Under System => Config => HA configure the Active-Passive Mode. In the Cluster Settings you can give a Group Name and a Password. In the port list you have to select the Heartbeat Interface. The Heartbeat Interface is the HA-Uplink.
Second Step on the Backup Firewall:
Make the same configuration on the Backup Firewall. But you have to give a less priority. Now switch off the Slave Firewall
Take a cross-over Cable for the HA-Ports and switch on the slave Firewall. Now check the HA Configuration on the Firewall.
Information about the Heartbeat Interface from Fortinet:
The FGCP heartbeat operates on TCP port 702. The time interval between HA heartbeats is 200 ms. The IP address used for the HA heartbeat (10.0.0.1, 10.0.0.2 etc) is an independent IP address not assigned to any FortiGate interface. You can view HA heartbeat sessions from the web-based manager System > Status > Session page. HA heartbeat sessions appear as TCP sessions between the HA heartbeat interface IP addresses that use port 702 as the destination port.