What happens if I enable both the otp and u2f apps? Can users then choose which way the want to authenticate or will it make them authenticate with one, then the other. Or can it be set up for either/or. So if I have a yubikey then I won’t need my otp but if I don’t have my yubikey then I can use otp instead?
You can use both at the same time. There will be a choice at the login window, to use either one.