Asynchronous multiple requests 401 Unauthorized

Apparently, this already is a known issue on GitHub. Still open and not patched from May 16, 2016.
Based on the suggestion from the comments, creating a new Hooks instance and registering the hook curl.before_multi_add with a callback that sets the cURL authentication options should solve the [401] Unauthorized Access problem.

use WpOrg\Requests\Hooks;

$hooks = new Hooks();
$hooks->register( 'curl.before_multi_add', function( &$curl_subhandle ) {
        // curl_setopt can fail => Still [401] then, so take care of that
        curl_setopt( $curl_subhandle, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
        curl_setopt( $curl_subhandle, CURLOPT_USERPWD, YOUR_CK . ":" . YOUR_CS );
    });

Afterwards, set the following in the $options array:

$options []= 'hooks' => $hooks;