Validate handler for field_test_entity_nested_form().
function field_test_entity_nested_form_validate($form, &$form_state) {
$entity_1 = entity_create('test_entity', $form_state['values']);
field_attach_extract_form_values($entity_1, $form, $form_state);
field_attach_form_validate($entity_1, $form, $form_state);
$entity_2 = entity_create('test_entity', $form_state['values']['entity_2']);
field_attach_extract_form_values($entity_2, $form['entity_2'], $form_state);
field_attach_form_validate($entity_2, $form['entity_2'], $form_state);
}