In order to use multiple database connections in your CodeIgniter project, you just need to create multiple configuration arrays that simplify working with multiple databases.

The Default Configuration Array

Following is the structure of the default Codeigniter database configuration array:

$db['default']['hostname'] = 'localhost';

$db['default']['username'] = 'root';

$db['default']['password'] = '';

$db['default']['database'] = 'cloudwaysdb';

$db['default']['dbdriver'] = 'mysql';

$db['default']['dbprefix'] = '';

$db['default']['pconnect'] = TRUE;

$db['default']['db_debug'] = FALSE;

$db['default']['cache_on'] = FALSE;

$db['default']['autoinit'] = FALSE;

$db['default']['stricton'] = FALSE;

$db['default']['cachedir'] = '';

$db['default']['char_set'] = 'utf8';

$db['default']['dbcollat'] = 'utf8_general_ci';

$db['default']['swap_pre'] = '';

So in order to create another database connection, you should create another configuration array. This array has to follow the same structure. Here is an example of the array:

$db['anotherdb']['hostname'] = 'XXX.XXX.X.XXX';

$db['anotherdb']['username'] = 'another_user';

$db['anotherdb']['password'] = '';

$db['anotherdb']['database'] = 'anothercloudwaysdb';

$db['anotherdb']['dbdriver'] = 'mysql';

$db['anotherdb']['dbprefix'] = '';

$db['anotherdb']['pconnect'] = TRUE;

$db['anotherdb']['db_debug'] = FALSE;

$db['anotherdb']['cache_on'] = FALSE;

$db['anotherdb']['cachedir'] = '';

$db['anotherdb']['char_set'] = 'utf8';

$db['anotherdb']['dbcollat'] = 'utf8_general_ci';

$db['anotherdb']['swap_pre'] = '';

$db['anotherdb']['autoinit'] = FALSE;

$db['anotherdb']['stricton'] = FALSE;

Read full article at: https://www.cloudways.com/blog/connect-multiple-databases-codeigniter/