No, it doesn't override. Also, N-400 extension also doesn't extend I-751 extension. Letters only extend physical GC expiration.
As of today, I-751 extension letters are 48 months and N-400 extension letters are 24 months. This means, for anybody with pending I-751, their I-751 receipt is much more useful than N-400 receipt when it comes to travel or proving status. N-400 extension letter is useful for somebody who has 6-12 months left on their 10 year GC and they don't want to file I-90.