@marl_scot
The networks on different interfaces must not overlapping.
And I don't know any router which is capable to route with that settings.
Maybe the ISP can give some recommendations.
Two IPs within the same subnet with the same gateway is not a real failover set up for my understanding.
If the ISP refuses to change one of the subnets your only one option might be to put a router between the ISP and pfSense and nat the traffic to a different subnet.