ho risolto da solo
ecco il code giusto:
function login($username,$pass)
{
GLOBAL $db,$table,$table1;
$username = trim($username);
$pass = trim($pass);
$query = mysql_query("SELECT * FROM $table tutorial_user_auth WHERE userName = '$username' AND userPassword = '$pass'");
if (mysql_num_rows($query)== " ") {
$query = mysql_query("SELECT * FROM $table1 admin_user_auth WHERE userAdmin = '$username' AND passAdmin = '$pass'");
return mysql_num_rows($query);
}
else {
return mysql_num_rows($query);
}
}