It seems that, in fact, I did miss a nuance mentioned on p. 71 of the article that I linked to in the question.
The only requirement in the Gemara concerning Shabbos Torah readings
is limited to the need to read certain portions of the Torah prior to
Shevuos and Rosh Hashanah, :מגילה לא׃ תניא ר׳ שמעון בן אלעזר אומר עזרא
תיקן להן לישראל שיהו קורין קללות שבתורת כהנים קודם עצרת ושבמשנה תורה
קודם ראש השנה
The rule states that the "curses" located in Bechukotai should be read before Shavuot. It is unclear if this means immediately before Shavu'ot, or by the time Shavu'ot comes, it should have already been read, meaning that it can be read 1 0r 2 weeks earlier.
Rambam תפילה יג׃ב after quoting the above Gemara almost verbatim,7
adds:
...והמנהג הפשוט שיהו קוראין במדבר סיני קודם עצרת. ואתחנן אחר תשעה באב.
אתם נצבים קודם ראש השנה. וצו את אהרן קודם הפסח בשנה פשוטה. לפיכך יש
שבתות שקורין בשחרית שני סדרין כגון אשה כי תזריע וזאת תהיה.ובהר סיני
ואם בחוקותי וכיוצא בהן כדי שישלימו בשנה ויקראו אותן הסדרים בעונתן
So it seems, that Ramba"m insists that we leave 1 week between Bechukotai and Shavu'ot. It seems that this is the opinion that we follow.
I checked the calendar arrangement in MS Outlook comparing this year with next year, as Pesach is on Shabbat in both years - this year regular year and next year a leap year.
This year, a non-leap year, if we (Diaspora) didn't double them up, Bechukotai would be read on Erev Shavu'ot, which is the Shabbat immediately prior to Shavu'ot (Shavu'ot is on Sunday.) Therefore, this parsha is doubled.
Next year, which is a leap year, in Diaspora, Bechukotai which is read on its own is already 2 weeks before Shavu'ot, and the Shabbat immediately before Shavu'ot is Bamidbar.
Ramba"m mentions an additional concern, namely that Va'etchanan be read after Tish'a B'Av. To do this, Devarim must be the parsha immediately before (or on) Tish'a B'av (as is this year. I think that's why Ramba"m was careful to phrase things in terms of Va'etchanan.) To do this, in Israel, Matot-Mas'ei are read separately. (In most years, they are doubled.)