db_set_active('mydatabase');    
    $query = db_select('mytable', 'c');
    $query->fields('c', array('number','email_address'))
    ->condition('number', $phoneval, '=')
    ->condition('mail', $mailval, '=');
    $result = $query->execute()->fetchAll();
You could use nested if
   
    if (empty($result)) {
      // do print_r($result);
      // here you will get in array
       if (empty($result[0])) {
        form_set_error('phone_number','We have no record of that Assessment Number');
       } else if(empty($result[1])) {
       form_set_error('mail','We have no record of that email');
       }
    }