I thought 2 days of rosh hashana was because of "niskalklu Haleviyim Bshirom" which clearly was at the time of the Bais Hamikdosh, not so much later like the 12th or 13th century.
thx
md
True, there were two days already from the times of the Beis ha'Mikdash as you said. However, there are two opinions about keeping two days of Rosh Hashanah in Eretz Yisrael after the calendar was set and witnesses are not used. Does Eretz Yisrael (not Chutz la'Aretz) revert back to one day? See Beitzah 5a-b and the Rosh (1:4) there who cites the two opinions regarding whether one day is enough. The Rif and Rosh require two days, while Rabeinu Efrayim and the Ba'al ha'Me'or require one. (Since the Gemara says that in the presence of the Beis Din that declares Rosh Chodesh, one day can be enough, and since such a Beis Din can function anywhere in Eretz Yisrael, it follows that one day is sufficient. See there for more discussion on this topic0.) We rule like the Rif and observe two days even in Eretz Yisrael.
All the best,
Reuven Weiner