During a regular (non-leap) Jewish year, there are four instances where two of the weekly Torah portions are read together in the same week. This is because there are not enough weeks to read all of the portions separately (especially because we do not read a weekly portion during Shabbos of Pesach and Sukkos). Why were those eight specific portions chosen for doubling?
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The Parshoyos that are sometimes doubled are: Vayakhel-Pekudei, Tazria-Metzora, Acharei-Kedoshim, Behar-Bechukosai, Chukas-Balak, Matos-Masai, Nitzovim-Vayelech.
The Gemara in Megila 31b says that Ezra established that the curses of Chumash Vayikra should be read before Shavuos and the curses of Chumash Devarim should be read prior to Rosh Hashana.
The Rambam in Tefila 13:2 mentions that the Minhag is to read Bamidbar before Shavuos, Veschannan after Tisha B'Av, Nitzavim before Rosh Hashana, and on a year with one Adar - Tzav before Pesach.
Although there may be other reasons - it seems that the Parshios selected are more connected in content than the others.