I think it is a great idea to have two EWP's. However, with you set-up. you move the single point of failure away from the pump and to the flapper valve. If the flapper dislodges, it could block the flow entirely.

Have you considered running one pump per radiator? If a pump fails, the cooling would be cut in half, but could still be enough to operate at reduced power. Also, the flow would reverse though the "dead" radiator. This would still provide some cooling, so the total cooling would be reduced by somewhat less than one half.

Of course you know that by installing two pumps, you double the chance of a pump failure.