Validate input of two possible arrays

You can set an custom callback on whatever field you want. For e.g. on the group_id. And in that custom callback grab both arrays and do your checks. Post are always available (in controller and model).

PHP Code:

public function check_groups()
{
    $add_to_group = $this->input->post('add_to_group');
    $remove_from_group = $this->input->post('remove_from_group');

    if($checks_valid)
         return true;

    return false;
} 


And depending on what fails, send custom error codes.

PHP Code:

$this->form_validation->set_message('check_groups', 'Add to group error');
$this->form_validation->set_message('check_groups', 'Remove to group error');
$this->form_validation->set_message('check_groups', 'No group selected?!');