This Shabbos is the second day of Shavuos. In Israel, however, it is Shabbos parshas Naso. We will not “catch up” to them until we read Chukkas-Balak. Why do we wait until Chukas Balak and not lain Naso-Behaaloscha together (other than we at the lainers union would strike for a parsha over 325 psukim long)? And, what is a ben chutz-laaretz to do if he’s in Israel for shuvuos?
We read the entire Torah every year, between Breishis and Simchas Torah, with the reading we read every Shabbos morning. See Rambam Hil. Tefilla 13:1; the fact that we read on Shabbos morning is in Halacha 3 there. There used to be a minhag to finish the Torah in 3 years, but we are not allowed to do that anymore. (Why the cycle begins on and ends based on Simchas Torah – see R. Zevin, Moadim B'Halacha, Simchas Torah. Seems to be from gemara in Megilla 31 that the reading of that day is Zos Habracha, but it is different than the other readings mentioned there - those are deviations from the order, this one is part of the order of what we read every Shabbos.)
Megilla 31b: R. Shimon ben Elazar said, Ezra decreed for Yisrael to read the curses in Vayikra (i.e., Bechukosai) before Shavuos and the curses in Devarim (i.e., Ki Tavo) before Rosh Hashana. Rambam adds quite a bit in Hil. Tefilla (13:2):
עזרא תיקן להן לישראל, שיהיו קורין קללות שבספר ויקרא, קודם עצרת; ושבמשנה תורה, קודם ראש השנה. והמנהג הפשוט, שיהו קוראין "במדבר סיני" קודם עצרת; "ואתחנן" אחר תשעה באב; ו"אתם ניצבים" קודם ראש השנה; ו"צו את אהרון" קודם הפסח בשנה פשוטה. לפיכך יש שבתות שקורין בשחרית, שני סדרין כגון "אישה כי תזריע" ו"זאת תהיה" ו"בהר סיני" ו"אם בחוקותי" וכיוצא בהן--כדי שישלימו בשנה, ויקראו אותן הסדרים בעונתן.
His main addition is that the minhag is to read certain parshios before holidays. (The Rav gave reasons for these; the only one I heard (from R. Chaim Ilson) is that Tzav is before Pesach because Tzav discusses koshering keilim that had hetera bala, and chametz before Pesach is heteira bala. I looked and looked and found this buried in Harerei Kedem.)
The Meiri in Kiryat Sefer, Maamar 5 chelek 1 gives a very good summary of the order of laining. Basically, there are more parshios than non-Yom Tov Shabbos days in the year, so to finish the Torah in a year, we must read some parshios together. He lists which ones may be read together. It seems to be fully based on minhag.
But what is clear from the Meiri, and seems to follow from the Rambam, is that we read two parshios together right before a “deadline parsha,” that is, a parsha which must be read before a given holiday. Tzav is read before Pesach in a non-leap year. It must be Tzav alone, not with Vayikra. And it does not logically make sense to read Pekudei and Vayikra together for two reasons: one is there is no such minhag, and two, there is a huge space in a sefer Torah between sefaim, so it does not make sense to read one parsha from sefer Shemos and one from Sefer Vayikra together. So we read Vayakhel Pikudei together. Why not Ki Sisa with Tetzaveh? The Meiri says Ki Sisa and Vayifen (about the Eigel) were two separate parshios which were kind of merged as one, so that’s considered a double parsha, and we read Terumah Tetzaveh before Ki Tisa, which allows us to reach Tzav the week before Pesach. We see that we only read double parshios as late as possible before the holiday. We don’t read Breishis & Noach together, and Lech Lecha and Vayeira together and then read all single parshios until Pesach.
So for Shavuos, we read Bamidbar before Shavuos. The next Shabbos we have to reach a certain Parsha for a certain holiday is Vaeschanan after Tisha B’av (as Rambam says). Again with a calculation similar to the one made for the parshios before Pesach, and with a further requirement (I don’t know why, but this is the minhag) we read the 3 haftoras of Destruction before Tisha B’av on Devarim and the two preceeding shabbasos – either Pinchas and Matos-Masei, or Matos and Masei alone, depending on which day of the week the fast of 17 Tammuz falls. So the last Shabbos before Pinchas is…a need to read Chukkas and Balak together. And with the regular rules of reading double Parshios, we read them as late as possible, and not Naso-Behhaloscha together, and in Chutz Laaretz we do not care that we are a week behind Israel’s parshios.