Study Rosh Hashanah folio 21A with parallel Hebrew-English text, traditional commentary, and modern study tools. Free access to Babylonian Talmud online.
The Talmud answers: It is a rare occurrence that two consecutive months are made short, and this would generate publicity, so that everyone would know about it.
§ It was related that Levi once arrived in Babylonia on what was observed there as the 11th of Tishrei. He said: How tasty is the dish of the Babylonians on the great day of Yom Kippur, as they are observing Yom Kippur in the West, Eretz Yisrael. The month of Elul had been declared full in Eretz Yi
It was further related that R' Yoḥanan used to proclaim: Anywhere that can be reached by the messengers who go out in Nisan in time to inform the people when to observe Passover, but cannot be reached by the messengers sent out in Tishrei, let them also observe the festival of Passover for two days.
It was reported that R' Aivu bar Naggarei and R' Ḥiyya bar Abba, two disciples of R' Yoḥanan, once arrived in a certain place that could be reached by the messengers who go out in Nisan, but could not be reached by the messengers who go out in Tishrei. And they saw that the locals observed only one
§ The Talmud relates that Rava would regularly sit in observance of the fast of Yom Kippur for two days, in case Elul had been declared a thirty-day month and Yom Kippur should be observed on what was observed in Babylonia as the 11th of Tishrei. It once happened in accordance with his opinion. Elu