Root and the password are the same

I don't believe that's possible. You could have two entries in /etc/passwd with the same user names but different UIDs, but the system would probably just ignore the second one (or misbehave in some way); arguably such an /etc/passwd file would be considered corrupt.

When you login to the system, you're first prompted for your user name. Once you've done that, the system prompts for your password, and checks whether the entered password matches the password for the account corresponding to that user name. By the time you're entering your password, the system has already determined what account you're trying to access.

I suppose you could modify various pieces of the system to get the behavior you want, but you'd have to replace several different pieces of software, including anything that authenticates and authorizes users (console login, su, ssh, and whatever other methods are enabled). Any mistakes would likely open huge gaping security holes.

EDIT : Based on the comments, PAM is probably the way to do this. I'm not familiar enough with PAM to go into more detail. (It's still a really bad idea.)

Verbatim:

In Mac OS X, keychain files are stored in ~/Library/Keychains/, /Library/Keychains/, and /Network/Library/Keychains/,

source: http://en.wikipedia.org/wiki/Keychain_(Apple)

edit Sorry, I didn't understand that you were asking for the login password...

For that, there is this similar question elsewhere, with a good answer...