Many shitos in this piece (and nearly all the scenarios) are brought by Rav Tzvi Hirsch Grodzinsky in Sefer Mikraei Kodesh Chelek Aleph, here on Hebrewbooks.org. (He was a cousin of R' Chaim Ozer Grodzinsky. He came to America in 1891 and served as Rabbi of Omaha, Nebreaska for 56 years, until his death in 1948. This sefer is a very comprehensive work on Kriyas Hatorah and everything related to it. His other seforim - on hilchos Mikvaos and Gemara Brachos are thankfully available at Hebrewbooks.org. They are superb sefarim.) See Klal 13 & Klal 19 for the shitos he brings on this topic (note in 19 he forgot to list Terumah Tetzaveh as a double pasha). I will refer to this sefer by MK and give klal number and seif and note numbers.
There is another time we read two parshios - if there was no minyan one shabbos in a community, the next week they read the missed parsha and the current week's parsha (see MK 13 ff 19 for a very interesting but brief discussion whether from takanas Moshe would allow them to read without a minyan). The revi'i aliya must connect the two parshios. If some shuls had a minyan and one didn't, they would only read the current week's parsha. This is because the parsha was read in the community, and kriyas hatorah is only an obligation on the community, not on an individual. Rav Chaim Soloveichik held it was an obligation of the individual based on the Milchamos Hashem (Ramban) Megilla first perek (read it very closely. Some, including Rav Chaim's mechutan Rav Elya Pruzhener and his grandson Rav Yoshe Ber Soloveitchik did not see it in the Milchamos, but it is there. Look closely.) I believe his great grandson Rav Moshe explained the Rabbeinu Chananel at the beginning of Megilla the same way, but we don't pasken like that.
If they missed two weeks, would they make up both missed parshios or only one? Some say only one: Maharam Mintz 85 - only one because we never find reading 3 parshios on one shabbos, only 2. A different reason to only read the most recently missed parsha (Shaar Hatizyun [Mishna Brurah] 135:8 from Biur Hagra) is because it's like tashlumin of davening, where you can make up only the most recently missed tefila and no more. Note - if it's like tashlumin, then you get into an issue that tashliumin only works if you missed it not bemaizid. (I heard Rav Refael Yitzchak Reichman of Geula in 2000 quote a Tosefes Maaseh Rav 34 (in siddur Hagra, on the bottom of Maaseh Rav, where the Gra was in jail for four weeks and after he was released he had the tzibbur read all four parshios he missed. He explained that we know the Gra in Biur Hagra says something because it is the opinion of the Mechaber or Rama, but he himself disagrees with it. This is what R' Reichman suggested as a possibility in these 2 Gras.) Mishna Brurah 135:6 brings the Eliyahu Rabbah that you would read all the parshios you missed.
If the missed parsha was a double parsha, or it was only one parsha but the next week was a double parsha, the Maharam Mintz holds you only read this week's parsha, because we don't find reading 3 parshios on one shabbos. He doesn't say the first case explicitly, but there is a sevara even to him that you would read the second of last week's parsha with this week's parsha, that way you only read 2 parshios. I heard that Rav Yoshe Ber said the kriyas hatorah of a shabbos is the inyana diyoma of that shabbos. I'd suggest, with what Rav Yoshe Ber said or even without it, that when 2 parshios are read on one shabbos, it is one kriya that happens to contain 2 parshios. But you could combine it with the next week's parsha if you missed it. I'd suggest - if the double parsha was missed, like Acharei Mos and Kedoshim, followed by Emor, that we'd read 1st aliya: acharei mos, 2nd - continue and begin Kedoshim; 3rd - continue Kedoshim; 4th - Kedoshim and continue into Emor (this is key, because a double parsha has to be linked in the fourth aliya. I'm suggesting Acharei Mos and Kedoshim are considered one, and Emor is the second), and continue 3 more aliyos in Emor. And this would be even simpler is we missed a double parsha and the next shabbos was also a double parsha - like Tazria & Metzora followed by Acharei Mos & Kedoshim - connect metzora & Acharei Mos in revi'i.
If it is a minyan that only meets once a month (by design, like in very small communitites), they probably aren't considered a set minyan and would not have to make up missed parshios; they'd only read the current week's parsha.
If the missed week was a chazak (like Vayechi or Pikudei etc.) most poskim say you don't make it up because it doesn't make sense to combine 2 parshios in the Torah that have a four blank lines in between them (MK 13 se'if 3 & ff 6). A weak sevara could be offered for the Aruch Hashulchan that you can read them, from Parshas Behaaloscha: Vayehi Binsoa is separated from the rest of the parsha with upside down noons (I didn't like how it looked when I spelled it "nuns".) In fact, some say Vayehi Binsoa is a separate sefer, and what we call Bamidbar is 3 books: Bamidbar until Vayehi binsoa, then vayehi binsoa is its own book, and after vayehi bonsoa unti ldevarim is another sefer. Yet we read Behaalosch, which includes readings from all three of those sefarim, in one kriya. A very weak proof, but interesting.