Validation Callbacks!!!

[eluser]reset[/eluser]
Hello all!!

I have a form with two multiple list. I move options from one multiple to the other one in order that the user want. I must validate that first multiple is empty and the other one have all the options...

How I can do that? I do this, but doesn't work, sorry for my english:

Code:

$rules['grupos1'] = 'empty';
$rules['grupos2'] = 'required';
//or
$rules['grupos1'] = 'callback_empty';
$rules['grupos2'] = 'required';
//don't work

$grupos_1 = $this->input->post('grupos1');
if (empty($grupos_1)) echo 'TRUE';
//print TRUE

Thanks