I haven't used the dbutil class before, but can't you just add the new database in your config/database file and load it? ie.
$db['new_db_name']['hostname'] = "localhost"; // localhost if same server
$db['new_db_name']['username'] = "un"; // your db username
$db['new_db_name']['password'] = 'pass'; // your db password
$db['new_db_name']['database'] = "new_db_name"; // your db name
$db['new_db_name']['dbdriver'] = "mysql";
$db['new_db_name']['dbprefix'] = "";
$db['new_db_name']['pconnect'] = TRUE;
$db['new_db_name']['db_debug'] = TRUE;
$db['new_db_name']['cache_on'] = FALSE;
$db['new_db_name']['cachedir'] = "";
$db['new_db_name']['char_set'] = "utf8";
$db['new_db_name']['dbcollat'] = "utf8_general_ci";
Then $this->load->database('new_db_name');
//whatever dbutil functions...
EDIT:
I just tested this and it seems the dbutil class uses the $this->db object. So if you set the result of $this->load->database('new_db_name', TRUE)
to $this->db
(second 'TRUE' parameter to return db object) then it works, ie:
$this->db = $this->load->database('new_db_name', TRUE);
$this->load->dbutil();
// do whatever