I don't recall the exact location Rambam's Mishneh Torah, but he discusses that during the time of Sanhedrin, if witnesses came before sundown of the 30th day, then that day was considered Rosh Hodesh, and the previous month was 29 days long. Otherwise, the 30th day was observed as Rosh Hodesh and the next day was considered the 1st day of the new month. Thus, there were 2 days Rosh Hodesh.
In the current fixed calendar, this is emulated. A month having 29 days has 1 day Rosh Hodesh which is the 1st of the next month. Months having 30 days have Rosh Hodesh on the 30th as well as the 1st day of the next month.
Rosh Hashannah is also Rosh Hodesh. If I understand the Sanhedrin rules correctly, the same situations occurred on Rosh Hashanna as with every Rosh Hodesh - i.e. the 30th of Elul was the 1st day of Rosh Hashanna, and Tishrei 1 was the 2nd day of Rosh Hashannah.
On the current fixed calendar, Elul has 29 days, and the 2 days of Rosh Hashannah are 1 and 2 Tishrei. Why are we not emulating how it works for all other months, and make Elul 30 days iwth Rosh Hashannah being on 30 Elul and 1 Tishrei?