I heard if a person in wajib Namaz, recited Surah Falaq he must recite Sura Naas, vice versa, is this true?

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It is not correct Both are complete surah and one is enough
But there are few surahs known as half surah and other surah has to be recited to complete
Surah Dhuha surah no 93 and surah Inshirah surah no 94.
Surah Al Fil surah no 105 and surah Quraish surah no 106.