In addition to what you're saying, I also would like to register multiple keys to services such that any one would work, not that both are required. I don't need them to be nuclear keys... I want a backup key stored in a safe and one on my keychain.
This seems to be very uncommon amongst service providers.
I’d say this is actually the most common. I’m not aware of any websites that allow >1 U2F/FIDO key in the configuration where you need to use all-of-them to log in. Sites either only support 1 key, or support multiple keys and you need 1 of them to log in.
Yep, I recently got Google's Titan pair of keys, and anywhere that supports hardware keys, I was able to register both, and I'm only required to use one to login.
This seems to be very uncommon amongst service providers.