There are trade-offs here, sometimes we might decide to take a very small risk, after weighing it up. A famous example is ETOPS. Unlike propeller engines, jet engines for airliners are ridiculously reliable once running. ETOPS (jokingly "Engines Turn Or Passengers Swim") is a scheme which over the past few decades has gradually allowed twin engine jets (which obviously just become over-engineered gliders if both their engines fail) to fly over oceans and other large distances with no airports, on the rationale that although total engine failure would probably be deadly without anywhere to put the plane down, two unrelated sequential engine failures are now so unlikely as to be overall negligible risk to the passengers.
But that needs a risk we understand. If you have no idea what the risk is, you can't make that trade, so redundantly safe systems are the obvious choice.
If one system detects an obstacle, stop the car. Even if this system was failing and the other wasn't detecting anything.
And if one system fail, yes the car should stop if it's not safe to drive with only one of them.
Either your eyes or ears may fail you now and again, but you're better off with both.
No?