Hello,
I'm using two autocomplete fields for free tags, and I made form validation, if both are not empty i display message to the user.
and its working fine the only problem is that the tags i wrote inside the fields are saved on both vocabulary this should not happen since the node is not stored, so terms are created even if there is no node!!

here is the code:

function mymodule_form_validate($form, &$form_state) {
if ($form['field_one']['value']['#value'] == ''" && $form['field_two']['value']['#value'] == ''") {
form_set_error('field_one', t('Please enter one of the following fields'));
form_set_error('field_two', ' ');
}
if ($form['field_one']['value']['#value'] != '' " && $form['field_two']['value']['#value'] != ''") {
form_set_error('field_one', t('Please enter one of the following fields'));
form_set_error('field_two', ' ');
}
}

Regards
Rami