I want to utilize TWO Yubikeys so that I have a backup incase one is lost/stolen/breaks/etc. This is a VERY standard practice when using physical security devices like Yubikeys and as far as I've found, PayPal is the only service I'm using at the moment that hasn't allowed me to pair a a second YubiKey with my login for 2 factor authentication means. 

Am I missing something? Is there in fact a way to utilize a second key? I click on "add new device" just like before, but now I'm only presented the option of adding a phone number for SMS verification instead of the ability to pair a second physical security device...