The premise.  You have a FortiGate and you have two ISP connections.  Maybe the speeds aren’t that close together and one of them is really just for failover so load balancing is out but you want BOTH WAN connections to respond from the outside.

It’s pretty straight forward.  Configure your IP addressing on both WAN interfaces.  Let’s assume WAN1 is going to be the primary connection.

The secret sauce is in the Distance and Priority for your static routes.  You want your distances to match but your priority for the primary connections to be lower.

Head over to Network > Routes and add a static route for your primary WAN, WAN1 for this example.

Now configure a static route for your backup connection, WAN2 for this example.

Now if both your links are active, both static routes will show up in the routing table (Monitor > Routing), your primary connection will be preferred for outbound traffic but inbound requests made to the secondary WAN connection will now respond correctly.

Of course don’t forget to configure your NAT’s and policies for both WAN connections!

Also, check out this post on adding a link monitor:

FortiGate WAN Link Monitor v5.4