form-snip2.php

// this will not work

$form['whitelist'] = array(
  '#type' => 'checkbox',
  '#states' => array(
    // uncheck this box when the other checkbox is checked.
    'unchecked' => array(
      ':input[name="blacklist"]' => array('checked' => TRUE),
    ),
  ),
);

$form['blacklist'] = array(
  '#type' => 'checkbox',
  '#states' => array(
    // uncheck this box when the other checkbox is checked.
    'unchecked' => array(
      ':input[name="whitelist"]' => array('checked' => TRUE),
    ),
  ),
);