If you need to connect to more than one database simultaneously you can do so as follows:
$DB1 = $this->load->database('group_one', TRUE);
$DB2 = $this->load->database('group_two', TRUE);

default database
$db['default'] = array( 'dsn' => '', 'hostname' => 'localhost', 'username' => 'root', 'password' => '', 'database' => 'test', 'dbdriver' => 'mysqli', 'dbprefix' => '', 'pconnect' => FALSE, 'db_debug' => (ENVIRONMENT !== 'production'), 'cache_on' => FALSE, 'cachedir' => '', 'char_set' => 'utf8', 'dbcollat' => 'utf8_general_ci', 'swap_pre' => '', 'encrypt' => FALSE, 'compress' => FALSE, 'stricton' => FALSE, 'failover' => array(), 'save_queries' => TRUE
);
another db
$db['database2'] = array( 'dsn' => '', 'hostname' => 'localhost', 'username' => 'root', 'password' => '', 'database' => 'test_2', // Database name 'dbdriver' => 'mysqli', 'dbprefix' => '', 'pconnect' => FALSE, 'db_debug' => (ENVIRONMENT !== 'production'), 'cache_on' => FALSE, 'cachedir' => '', 'char_set' => 'utf8', 'dbcollat' => 'utf8_general_ci', 'swap_pre' => '', 'encrypt' => FALSE, 'compress' => FALSE, 'stricton' => FALSE, 'failover' => array(), 'save_queries' => TRUE );