I've never had twin outboards in the water, so I was envisioning running them onto the same ledge or into the same big log, but it sounds like that rarely happens. Perhaps it's that the object is rarely completely perpendicular to the centerline of the boat, so whatever it is, tends to hit one lower unit (first), it absorbs most of the energy, and thus protects the second one. At any rate, sounds like it is exceedingly rare to take out two, and most redundancy solutions (especially if they are big heavy ones) "should" probably be for things that are more likely to happen.