8 – Split custom form into multiple smaller functions

I’ve a large custom form. As per the coding standards the function should not be more than 150 lines. So I tried to split the form into multiple (2) functions. But that didn’t work.
Here is what I tried

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form('some_element') = (
       '#type' => 'textfield',
    );
    ..
    ..
    if ($some_condition_exists) {
    //include second form here.
    $this->secondForm($param);
    }
    return $form;
    }

  public function secondForm($param) {
       $form('some_element') = (
       '#type' => 'textfield',
       '#default_value' => $param
    );
    return $form;
 }

Do I really need to put this form into another Form file and call Drupal::formBuilder()->getForm('DrupalmoduleFormxxxForm'); ?