This sounds like a good idea at first even though it does have the side-effect of creating a lot of orphan password hashes in your table. Let's step into the shoes of a criminal that has access to your database and hashing algorithm and see if it makes his life that much more difficult.

The first thing he would do is a dictionary attack from a list of common passwords: Pick one user, create the hash and search the hash table for a match. That is one step more than only having to check the hash in the same row. I'd say it is slightly, but not significantly, more difficult.