Zwracanie wszystkich wierszy z określonymi warunkami

Jesteś blisko, proste UNION ALL powinien robić, co chcesz;

SELECT MAX(del_time), COUNT(*) cnt, MAX(subject) subject FROM (
  SELECT to_delete del_time, thread_hash, subject
  FROM messages
  WHERE to_user_id = 27 AND `to_delete` >= SUBDATE(NOW(), INTERVAL 720 HOUR)
  UNION ALL
  SELECT from_delete del_time, thread_hash, subject
  FROM messages
  WHERE from_user_id = 27 AND `from_delete` >= SUBDATE(NOW(), INTERVAL 720 HOUR)
) a
GROUP BY thread_hash;

Zmodyfikowany SQLfiddle do testowania .