Enqueue script/style with multiple GET parameters

esc_url is run on the stylesheet URL and that converts those characters. You can work around it with a couple of filters.

function style_params($src, $handle) {
  if ('twentyfourteen-style' == $handle) {
    add_filter('clean_url','alter_clean_url',10,3);
  }
  return $src;
}
add_filter('style_loader_src','style_params',10,2);

function alter_clean_url($good_protocol_url, $original_url, $_context ) {
  remove_filter('clean_url','alter_clean_url',10,3);
  $good_protocol_url = html_entity_decode($good_protocol_url);
  $good_protocol_url = $good_protocol_url.'&abc=def';
  return $good_protocol_url;
}