I find your lack of faith in out mechanical skills... disturbing.
The point is that many dont want redundancy, but bigger flow. I still belive that parallel gives a bigger flow that series.
Besides, a simple thing to do would be a flow meter from both pumps, or add a parallel circuit to the pump power wires. If one goes, it triggers a relay that closes a valve. Ok, so electrical valves are huge and expensive, but ... you only need to interrupt the flow of the dead pump in the outlet.
It can be done with a manual valve , a switch and a small 12v motor a one (yes, one) gear. Let's just say we had to improvise. It was a pneumatic system but the operating principle should be the same.