In this case, the Rambam's halacha is really directly from an explicit Gemara, Niddah 38a. The Gemara gives several examples of how a woman could go a certain number of consecutive days (150, 100, 25) of bleeding and not be a major zava. All of the calculations involve a 9 day sequence of seeing the last 2 days of the 11 interim days, followed by 7 days of niddah, followed by the first 2 of the next 11 interim days. The Gemara has 3 baraisos in which this happens. One example:
תניא היה ר"מ אומר יש מקשה ק"נ יום ואין זיבה עולה בהן כיצד שנים בלא עת ושבעה נדה ושנים של אחר הנדה
Tanya: Rebbi Meir would say a woman could have koshi for 150 days without Ziva coming into them. How? 2 days of "b'lo es" (paraphrase of the verse of zava, the 11 days between niddah cycles), 7 days of niddah, 2 days after niddah ...
There is an Amoraic dispute as to whether or not a woman who sees on days 10 and 11 of the 11 interim days needs shimur, the rationale of the opinion (Reish Lakish, Niddah 72b) that says she does not need shimur being that she cannot become a major zava through these two days, as they do not join with the following day, the first day of niddah, to make 3 days of zivah. Rashi there is clear that both opinions in that dispute agree that day 10 is not fit to combine to a major zava, the dispute is only if it causes a requirement of shimur. [Indeed, the Rambam who rules that days 10 and 11 do not combine to day 12 rules that day 10 still requires shimur (Hilchos Mishkav Umoshav 5:8).]
If you want an example of another Rishon who makes this point explicitly, Tosefos Niddah 37b s.v. ורב אדא cites from Rabbeinu Chananel:
ר"ח פי' דלוי אינו מטהר אלא תשעה ימים הראוין ליעשות בהן זבה אבל י' וי"א דאינם ראויין ליעשות בהן זבה דאפילו ראתה אפי' גם בי"ב אינה אלא נדה
Rabbeinu Chananel explains that Levi only purifies the 9 days during which she could become a [major] zava, but day 10 and 11, which are not able to become a zava on them, as even if she sees also on day 12 she is only a niddah...
