Today the backup practice is to enable 2 keys in all accounts: one that you keep with yourself, the other that you leave in a safe.

There's been some experiments of creating copies of the master secret, e.g. [1]. Today you can do so either w/ u2f zero or with its upgrade solo hacker (note the hacker version), but we currently don't support it officially.

My personal advice as of now is to always have security key(s) + totp code. The security keys protect you against phishing, so if you click on an email link and get prompted for login, you're either safe (if you use the security key) or at least reminded about the risk (if you're used to use the security key but you don't have it with you at the moment). Viceversa, if you're directly logging into a website and you typed the url yourself, then totp offers the same security, so it's a totally valid alternative. Hope this makes sense.

[1] https://dmitryfrank.com/articles/backup_u2f_token