How can one reconcile Allah's saying in Surat Al-A'raf: "Indeed, your Lord is Allah, who created the heavens and the earth in six days and then established Himself above the Throne," with His saying in Surat Fussilat, which indicates that the creation of the earth and what is in it took six days, then He created the heavens in two days, making the total eight days?

There is no contradiction between the verses of the Quran regarding the duration of the creation of the heavens and the earth. His saying, the Almighty, in Surat Fussilat: "He created the earth in two days" and "He proportioned therein its sustenance in four days" means that the proportioning of sustenance took two additional days to complete the total number of days taken for the creation of the earth and what is in it to four days, not four independent days. Then He created the heavens in another two days, making the total six days, and this aligns with other verses that mention that creation was completed in six days.