we do not allow the change of screennames, to increase the "identification" with the name, and to help our moderators keeping track of insane users.
in that case, different login- and display-name just don't make sense, it even confuses some users, who don't understand why they can't login with their screenname.
I suggest that if changing the screenname is not allowed in general, newuser.php should just ask for one name only, the script could enter the value automatically in both fields, just in case you choose to allow it at a later time.
flint
flint@poisonfree.com