The second Callback is not called. I switched between them and got same result (the new second was not called)

$this->add($factory->createInput(array(
    'name'       => 'related_activity_id',
    'required'   => true,
    'validators' => array(
        array(
            'name' => 'Digits',
            'options' => array(
                'messages' => array(
                    'notDigits' => "No Related Activity ID was selected for update",
                ),
            ),
        ),
        array(
            'name'              => 'Callback',
            'options'           => array(
                'callback'      => array($model, 'isRelatedActivityConnected'),
                'messages'      => array(
                    'callbackValue'  => "The Related Activity is already connected to the current Activity",
                ),
            ),
        ),
        array(
            'name'              => 'Callback',
            'options'           => array(
                'callback'      => array($model, 'activityExist'),
                'messages'   => array(
                        'callbackValue'  => "The submited Related Activity ID does not exist",
                ),
            ),
        ),
    ),
)));