return [
    'components' => [
        'db1' => [
            'class' => 'yii\db\Connection',
            'dsn' => 'mysql:host=localhost;dbname=db1name', //maybe other dbms such as psql,...
            'username' => 'db1username',
            'password' => 'db1password',
        ],
        'db2' => [
            'class' => 'yii\db\Connection',
            'dsn' => 'mysql:host=localhost;dbname=db2name', //maybe other dbms such as psql,...
            'username' => 'db2username',
            'password' => 'db2password',
        ],
    ],
];
使用方法

//to get from db1
Yii::$app->db1->createCommand((new \yii\db\Query)->select('*')->from('tbl_name'))->queryAll()

//to get from db2
Yii::$app->db2->createCommand((new \yii\db\Query)->select('*')->from('tbl_name'))->queryAll()
或都在module里里定义

public function getDb() {
    return Yii::$app->db1;
}

//Or db2
public function getDb() {
    return Yii::$app->db2;
}


调用方法

ModelName::find()->select('*')->all()