Yes but since the client will only sent the hashed password with the salt to the server, you will need to use the same salt on the second database, if you want the hashed passwords to match.
@nexialy
Yes, this is possible ... however you only spoke about register.php.
Anyway, it's not recommended
I don't want a plaintext password to be pushed around. That's too much of a security risk I'm willing to accept. I'm just wanting to push a MD5 password around.
I know vBulletin created the MD5 clientside as well as the our current Hashing for vB3.x.