Presumably, the point which caused you to imagine that these two verses are incoherent and contradictory with each other was the translation of the word of “al-Samā” into “one heaven” whereas the real meaning of the word is “heaven” not “one heaven”. Even in the English translation of the word "al-Samā" which literally means "sky", there is no incoherence between the word “sky” and “skies”. For example, when a person says, “I saw cloud in the sky” or “I saw clouds in the skies”, both are conveying the same meaning; neither “cloud” is in contradiction with “clouds” nor is “heaven” with “heavens”. There is no incongruity between the singular and plural forms.
In order to reply to your question we should first know that the two sentences are evaluated as being contradictory with each other only when they meet all the conditions of a contradiction. For example if a person says “I offer my prayers in the mosque” and again he says “I attend congregational prayers held in several mosques", we cannot describe the two sentences as being contradictory with each other simply because he has used the singular form of a word in one sentence and the plural form in another.
Likewise, if we look carefully at the usages of the words in Arabic language, especially in the holy Quran, we will come to know that the Arabic word "al-Samā" which means “heaven”, is not so much different from “al-Samawāt” (heavens), because the word "al-Samā" does not mean "one heaven" otherwise it would have been in contrast with "al-Samawāt" (heavens). So the word "al-Samā" used in many different verses of the Quran implies the same meaning. For example when God, the Glorified, says in Holy Quran, "Wa maa khalaqna al-Sama wal ardha wa ma bainahoma batila"[1], it means "And We did not create the heaven and the earth and what is between them in vain". According to your translation, the meaning should be: We did not create “one heaven and the earth and what is between them in vain” (which implies that creating the other heavens should have been in vain!!).
Similarly, the verse "ma yakhfa 'alallahi min shay'in fil ardh-e wa la fis-Sama"[2] would be translated as such: “and nothing in the earth nor any thing in one heaven is hidden from Allah" which, by extension, means that He may not know what happens in the other heavens. Obviously, nobody, who is at home in Arabic, will accept such a translation.
Be sure that the word "al-Samā" in verse 21 of Chapter al-Hadid does not mean “one heaven” to be in contradiction with "al-Samawāt" in verse 133 of Chapter Aal-e Imran; they are congruous with each other. On many occasions, when the Holy Quran wants to mention a point in regards to some heavens, not all of them, it does not use the word "al-Samā" alone; rather the terms such as "al-Sama al-dunya[3] (the lower heaven), or "kollu sama"[4] (every heaven) have been used.
Of course, if it had been said, “Hasten to Paradise whose breadth is that of the lower sky”, it would imply incoherence with other verses which say that the width and breadth of the Paradise is that of the whole of the heavens but there is not any verse like this in the Quran.
All praise is due to Allah, the Lord of the worlds.