SD Wan is the best way

Dual ISP : Step-By-Step Guide: Configuring IPsec Over SD-WAN

Single ISP : Configure IPsec VPN with SD-WAN – Fortinet Community

Manual Redundant with Link Monitors Manual redundant VPN configuration | FortiGate / FortiOS 7.6.4 | Fortinet Document Library

Redundant IPSEC Tunnel using single WAN c… – Fortinet Community

IPsec VPN: Site-to-Site tunnel monitor – Fortinet Community

internet: Distance 10 Priority 0

Primary VPN tunnel: Distance 10 Priority 0

Backup VPN tunnel: Distance 10 Priority 5 (as long as the priority is higher than the other tunnel it will only be used if the primary tunnel goes down.) If the distance was set higher then 10, it would not be able to be used until the primary tunnel went down.

Then under the Phase 2 / Advanced: Enable Auto-negotiate for both tunnels to stay connected.