The two GRE tunnels can then be used for branch to Primary/Backup Main Office in either Redundancy or Load sharing mode:

Redundancy GRE2 is configured as a backup interface for GRE1, and is activated only when GRE1 is down

Load sharing Both Tunnel interfaces are active. Routing protocols (RIP or OSPF) route traffic to destinations based on route cost and availability, as follows:

For two routes of equal cost to the same destination, one through the Primary Main Office and one through the Backup Main Office, OSPF will automatically distribute traffic through both routes, effectively sharing the load between routes.