It’s often claimed that multi-factor authentication is inherently more secure than single-factor authentication, but if you look at the history of this claim,it actually came from a vendor that wanted to make their multi-factor authentication product sound better than competitors' products.(I'm not sure if these are his thoughts or if he's saying that was the consensus at a recent X9 meeting.)
Martin goes on to suggest that using two authentication mechanisms of the same factor may be as secure as using two factors and lays out scheme A & B to discuss.
So here's my thoughts:
Wouldn't scheme A be more secure because you can't brute force it? Isn't that the whole point of having the second factor? All passwords can be brute-forced given enough time. Having the second factor removes that threat.
Of course, you could implement a strong password plus a kill switch after 10 bad tries, but that still relies on the user to implement safe password storage. And I generally think it's better to remove any responsibility from the end-user (especially if there's a convenience trade off).
Requiring users to carry/remember two username-password combinations for every system doesn't seem practical. Security will fail if users try to subvert it for the sake of convenience. And they will.
Usability needs to be a key consideration. A token/pin combination is a secure and easy-to-use way to beat the threat of brute-force attacks and poor password management. ...as is having a certificate installed on a particular PC and other second factor solutions.