completely off.
you would need to create a second variable and edit the SQL.
$MM_dupKeyRedirect="../nameused.php";
$loginUsername = $_POST['redfield'];
$loginUsername2 = $_POST['bluefield'];
$LoginRS__query = sprintf("SELECT RESTRCTR FROM dl_rstrctr WHERE RESTRCTR=%s OR RESTRCTR=%s", GetSQLValueString($loginUsername, "text"), GetSQLValueString($loginUsername2, "text"));
mysql_select_db($database_Xconn, $Xconn);
$LoginRS=mysql_query($LoginRS__query, $Xconn) or die(mysql_error());
$loginFoundUser = mysql_num_rows($LoginRS);