How do I fail an Internet connection between two sites

My setup is two physically separate sites connected via the core switch's at each site by an BT RAO1 connection.

Each site also has a pair of FortiGate 600D firewalls as HA pairs.

Both pairs have ISP WAN connections.

My aim is, if site A's internet connection fails, the FortiGate recognises this and routes the traffic back into the LAN and across the RAO1 circuit and out of site B's firewalls and vice versa.

Is this possible and if so how would I achieve it?

Many thanks in advance.