I would think that the most likely failure mode would be bird ingestion. A large bird would likely break the top prop, and the fragments of that prop+bird woul

I would think that the most likely failure mode would be bird ingestion. A large bird would likely break the top prop, and the fragments of that prop+bird would hit the cowel, and then be sucked down through the bottom prop. Not to mention the high forces exerted on the top-prop motor mounting from losing only one blade. I'm not sure whether you want the top prop motor mount to fail at that point to save the airframe, but if so, then you'll get the top motor ingested into the bottom prop too. Unless you can actually prevent large bird ingestion, or design props that can survive it intact, I agree you don't really have any redundancy from co-axial contra-rotating props.