Disable First Sub-Field of multi-value field

That can be done with hook_field_widget_single_element_WIDGET_TYPE_form_alter.

/**
 * Implements hook_field_widget_single_element_WIDGET_TYPE_form_alter().
 */
function HOOK_field_widget_single_element_double_field_form_alter(array &$element): void {
  $element['first']['#disabled'] = TRUE;
}

Thanks, Chi!