Create an advanced search that searches only on a field with search_api in drupal 7
You don't need separated indexes. In you callback handler, you need to build a new SearchApiQueryInterface instance and set conditions on the field using its condition method.

$index = search_api_index_load('custom_index_machine_name');
$query = $index-&gt;query(array(
  'language' =&gt; $GLOBALS['language']-&gt;language,
  'search id' =&gt; 'custom-search',
));
foreach($form_state['values']['filters'] as $field =&gt; $value) {
  if (!empty($value)) {
    $query-&gt;condition($field, $value);
  }
}
$result = $query-&gt;execute();