Although these two Surahs(Nas+Falaq) of the Qur'an are separate entities and are written in the Mushaf also under separate names, yet they are so deeply related mutually and their contents so closely resemble each other's that they have been designated by a common name Mu'awwidhatayn (the two Surahs in which refuge with Allah has been sought). Imam Baihaqi in Dala'il an-Nubuwwat has written that these Surahs were revealed together, that is why the combined name of both is Mu'awwidhatayn.
**Period of Revelation
Hadrat Hasan Basri, 'Ikrimah, 'Ata' and Jabir bin Zaid say that these Surahs are Makki(revealed in Mecca). A tradition from Hadrat 'Abdullah bin 'Abbas also supports the same view. However, according to another tradition from him, it is Madani(revealed in Madinah) and the same view is held also by Hadrat 'Abdullah bin Zubair and Qatadah.
**Theme and Subject-Matter
The conditions under which these two Surahs were sent down in Makkah were as follows. As soon as the Holy Prophet (upon whom be peace) began to preach the message of Islam, it seemed as though he had provoked all classes of the people around him. As his message spread the opposition of the disbelieving Quraish also became more and more intense. As long as they had any hope that they would be able to prevent him from preaching his message by throwing some temptation in his way, or striking some bargain with him, their hostility did not become very active. But when the Holy Prophet disappointed them completely that he would not effect any kind of compromise with them in the matter of faith…
**Full Introduction here: http://www.englishtafsir.com/Quran/114/index.html