I am new user of WP_User_Query(). I don’t know how to express whole situation. If I use below code I am getting good result.

$users = new WP_User_Query(array(
        'meta_query' => array(
            'relation' => 'OR',
            array(
                'key' => 'last_name',
                'value' => $_POST['search'],
                'compare' => 'LIKE',
            ),      
        )
    ));

If I use below code I am getting good result also.

$users = new WP_User_Query(array(
        'meta_query' => array(
            'relation' => 'OR',
            array(
                'key' => 'organisation',
                'value' => 519,
                'compare' => 'LIKE',
            ),          
        )
    ));

I am trying to combine these two query like below.

$users = new WP_User_Query(array(
        'meta_query' => array(
            'relation' => 'OR',
            array(
                'key' => 'last_name',
                'value' => $_POST['search'],
                'compare' => 'LIKE',
            ),
            array(
                'key' => 'organisation',
                'value' => 519,
                'compare' => 'LIKE',
            ),          
        )
    ));

But I am not getting expected output.

How can I combine these two queries ?