Fortinet FortiGate 1U Quick Start Manual page 225

Go to VPN > IPsec > Auto Key (IKE).
Select Create Phase 1 and create the
secondary tunnel. Set IP Address to use
FortiGate 2's wan2 IP, Local Interface
to wan2 (the secondary Internet-facing
interface) and enter the Pre-shared Key.
Go to VPN > IPsec > Auto Key (IKE).
Select Create Phase 2. Set it to use the
new Phase 1
Using redundant OSPF routing over IPsec VPN 205