When does a FortiGate load-share traffic between two static routes to the same destination subnet?

FortiGate static routes do not explicitly use a "cost" metric in the same way some dynamic routing protocols do; the primary metrics for selection and load balancing are distance and priority.

While "distance" is correct, "weight" is not the FortiGate metric used for load balancing in this context; FortiGate uses "priority" for static routes to determine load sharing behavior when distances are equal.

Having different priorities for static routes to the same destination with the same distance would mean that only the route with the lowest priority value (highest preference) would be used, not load-shared.

For FortiGate to load-balance traffic across multiple static routes to the same destination, all those routes must have identical administrative distances (which determines preference over other routing protocols) AND identical priorities (which determines preference among static routes of the same distance). When both are equal, load balancing occurs.