/**
 * Implements hook_preprocess_html
 */
function THEME_preprocess_html(&$variables){
  // Only add on node pages
  if($node = menu_get_object('node')){
    // Read node fields, wrapper requires entity api module
    $wrapper = entity_metadata_wrapper('node', $node);
    if(isset($wrapper->field_some_field)){
      if($some_content = $wrapper->field_some_field->value()){
        $element = array(
          '#tag' => 'html_tag',
          '#type' => 'meta',
          '#attributes' => array(
            'name' => 'some-metatag',
            'content' => $some_content,
          ),
        );
        drupal_add_html_head($element, 'some_metatag');
      }
    }
  }
}

function yourmodule_form_alter(&$form, &$form_state, $form_id){
  if( $form_id == 'your form id' ){

    $form['field_B']['#access'] = FALSE;
    $form['#validate'][] = 'yourmodule_validate';

  }
}

function yourmodule_validate($form, &$form_state) {

    $field_A_value = $form_state['values']['field_B']
    $form_state['values']['field_B'] = $field_A_value;

}