The Swiss cheese model is what people use to sell you more 'security' related software systems that inherently involve more problems. (Also cheese is not very durable, even the kind without holes.)
That is redundancy in my book. I don’t expect holes in my GNSS devices. And if you want to be sure, bring three, because two GNSS units with different readings are not very helpful.
I don't expect holes. But both devices are exposed--something could happen to one of them. And since I like going out in the middle of nowhere I assume I either have to get myself out, or if that's impossible summon help. I don't want a single point of failure on either of these.
Hiking with two GPS-capable devices is Swiss cheese.