Several sites/domains out there enable you to log in using any one of a few usernames.
Verizon Wireless, for example, permits you to log in using your phone number or your username.
In both cases, however, you are logging in using the same IDENTITY or "account".
It would be nice to enable us to tell the system "yes, I see that you think I am 'reusing' this password for two accounts - but they are REALLY the SAME account. Literally the same underlying identity"