As I understand it, it checks with the domain from the 'email address' to verify you. (air quotes because it needn't be possible to send email to the identifier

As I understand it, it checks with the domain from the 'email address' to verify you. (air quotes because it needn't be possible to send email to the identifier). So it wouldn't make sense to have more than one way to verify one ID. If the site you're signing into is particularly sensitive, it could require a separate check - perhaps even a second BrowserID login from a different domain.