I need to connect one more database.

how I suppose to configure default/config.php

 $config = array (
  'driver' => 'mysql',
  'host' => 'localhost',
  'database' => 'codoforum',
  'username' => 'root',
  'password' => 'root',
  'prefix' => '',
  'charset' => 'utf8',
  'collation' => 'utf8_unicode_ci',
);

I don't think that's possible. I guess there is only single database instance. This might require editing the core application. You need to create another database instance with another set of $config array. However querying with joins will be harder with an approach like that.

Can you tell me what is your reason to connect to another database and care to explain this some more?