'db' => require(__DIR__ . '/db.php'),
'db2' => [
    'class' => 'yiidbConnection',
    'dsn' => NULL,
    'username' => NULL,
    'password' => NULL,
    'charset' => 'utf8',
],

Yii::$app->db2->close(); // make sure it clean
Yii::$app->db2->dsn= 'yourdsn';
Yii::$app->db2->username = 'username';
Yii::$app->db2->password = 'password';

Yii::$app->db2->...

$connection = new yiidbConnection([
    'dsn' => $dsn,
    'username' => $username,
    'password' => $password,
]);
$connection->open();
$command = $connection->createCommand('SELECT * FROM post')->....;