call external form in ajax callback
You can use..

function custom_conditional_form($form, &amp;$form_state) {
  $form['form1']['drupal_love'] = array(
    '#title' =&gt; t('Do you love Drupal?'),
    '#type' =&gt; 'radios',
    '#options' =&gt; array('yes' =&gt; t('Yes'), 'no' =&gt; t('No')),
  );
  $form['form1']['how_much_love'] = array(
    '#title' =&gt; t('Great! Tell us more:'),
    '#type' =&gt; 'textfield',
    '#states' =&gt; array(
      'visible' =&gt; array(
        ':input[name="drupal_love"]' =&gt; array('value' =&gt; 'yes'),
      ),
      'required' =&gt; array(
        ':input[name="drupal_love"]' =&gt; array('value' =&gt; 'yes'),
      ),
    ),
  );
  $form['form1']['submit'] = array(
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Save'),
  );

  return $form;
}