It is possible ever since we launched this feature in 2016 with Akeeba LoginGuard. The Joomla! built-in feature in Joomla! 4.2 and later – what we now use on our site – is, in fact, Akeeba LoginGuard. I just contributed that code back to Joomla!. So, it's fairly easy :)
Go to My Account, My Profile from the top menu.
Click on Edit Profile.
Scroll down to Multi-factor Authentication.
Now, it depends on how you want to use the YubiKey.
A. For Yubico OTP (standard security)
Under YubiKey there's a button "Add a new YubiKey". Click on it to add another YubiKey.
B. As a WebAuthn authenticator (via the YubiKey's FIDO2 feature, maximum security)
Under "Web Authentication" there's a button "Add a new Web Authentication". Click on it to register another YubiKey as a WebAuthn authenticator.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!