This is default database :

$db['default'] = array(
    ....
  	....
    'database' => 'mydatabase',
    ....
);
Add another database at the bottom of database.php file

$db['second'] = array(
    ....
  	....
    'database' => 'mysecond',
    ....
);

In autoload.php config file

$autoload['libraries'] = array('database', 'email', 'session');

The default database is worked fine by autoload the database library but second database load and connect by using constructor in model and controller...

<?php
    class Seconddb_model extends CI_Model {
        function __construct(){
            parent::__construct();
            //load our second db and put in $db2
            $this->db2 = $this->load->database('second', TRUE);
        }

        public function getsecondUsers(){
            $query = $this->db2->get('members');
            return $query->result(); 
        }

    }
?>
  
/*
I hope it will help you.
Namaste
Stay Home Stay Safe
*/