Do last week's פרשה first, according to Halachapedia:
If one misses a week one should read the previous Parsha's Shenayim Mikra and then the current weeks Parsha. [46]
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[46] Sh”t Bear Moshe 5:79 writes that if one misses a week one should read the previous Parsha's Shenayim Mikra and then the current weeks Parsha. Rav Shlomo Zalman Halichot Shlomo (Tefillah 12:36), Ketzos haShulchan 72:9, Chazon Ovadyah (Shabbat, vol 1, pg 314), and Daily Halacha (by Rabbi Mansour) agree. However, Sh”t Yitzchak Yiranen 1:32 says one can make up a parsha out of order. Halichot Shlomo (Tefillah 12:37) adds that if one did begin the current week's parsha before one finished the previous one, one may finish it.