Can we make 2nd field required only if 1st field is filled in a custom composite field?
You can do it with two ways


With Webform Custom Handlers.
With custom js.


I prefer you to go with the 1st approach I mentioned above. How to create and work with webform custom handlers is documented well on drupal.org's webform document. Attaching here for quick reference.

namespace Drupal\mys_module\Plugin\WebformHandler;

use Drupal\Core\Form\FormStateInterface;
use Drupal\webform\Plugin\WebformHandlerBase;
use Drupal\Component\Utility\Html;
use Drupal\webform\WebformSubmissionInterface;
use Drupal\Core\StringTranslation\StringTranslationTrait;

/**
 * Webform validate handler.
 *
 * @WebformHandler(
 *   id = "my_module_custom_validator",
 *   label = @Translation("Alter form to validate it"),
 *   category = @Translation("Settings"),
 *   description = @Translation("Form alter to validate it."),
 *   cardinality = \Drupal\webform\Plugin\WebformHandlerInterface::CARDINALITY_SINGLE,
 *   results = \Drupal\webform\Plugin\WebformHandlerInterface::RESULTS_PROCESSED,
 *   submission = \Drupal\webform\Plugin\WebformHandlerInterface::SUBMISSION_OPTIONAL,
 * )
 */
class CustomValidatorWebformHandler extends WebformHandlerBase {

  use StringTranslationTrait;

  //We will overwrite the method validateForm, inherit from WebformHandlerBase.
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &amp;$form, FormStateInterface $form_state, WebformSubmissionInterface $webform_submission) {
    $this-&gt;validatePhone($form_state);
  }

  //We will define a new method to validate for example the phone.
  /**
   * Validate phone.
   */
  private function validatePhone(FormStateInterface $formState) {
    $value = !empty($formState-&gt;getValue('phone')) ? Html::escape($formState-&gt;getValue('phone')) : NULL;

    // Skip empty unique fields or arrays (aka #multiple).
    if (empty($value) || is_array($value)) {
      return;
    }
    if ($value &gt; 9) {
      $formState-&gt;setErrorByName('phone', $this-&gt;t('Phone not valid.'));
    }
    else {
      $formState-&gt;setValue('phone', $value);
    }
  }
}


Enable it for your webform: The route to administer the handlers of a Webform is  /admin/structure/webform/manage/[your_webform_name]/handlers

The second way is to add javascript in assets section of the webform:
Route is: /admin/structure/webform/manage/[webform]/settings/assets

In the case of jQuery, I suggest instead of removing the required try adding it. As the property is configured in the webform element setting.