The much bigger problem is that you also need to enroll a second Yubikey into every service you use, but also safely store that backup key somewhere, ideally outside your own house. That's just not realistic.

Yubico had some ideas around "paired Yubikeys" which shared the same root secret, but I believe that model won't work anymore with FIDO/CTAP2 due to some counter value that was added there over U2F. (It might be possible for them to just not have a security counter; I'm not sure.)