Fortinet FortiGate 1U Quick Start Manual page 231

Select Create Phase 1 and create the
secondary tunnel. Set IP Address to use
FortiGate 2's IP, Local Interface to wan2
(the secondary Internet-facing interface) and
enter the Pre-shared Key.
Select Create Phase 2. Set it to use the
new Phase 1.
Using redundant OSPF routing over IPsec VPN 211