Why do some months have 2 days for Rosh Chodesh and some just 1?
Initially, the New Month would be enacted by means of witnesses who would testify to the appearance of the new moon. If the new moon was spotted early enough, Rosh Chodesh (the New Month) would be declared for that day, and the previous month would have 29 days. If the new moon was not seen until later, Rosh Chodesh would only be on the next day, and the previous month would have 30 days.
Because of the possibility that the 30th day of the month will be Rosh Chodesh, it was customary to practice the customs of Rosh Chodesh on the 30th day of the month (because of the possibility that witnesses would come on this day; see Tashbatz 3:244).
This custom is continued today: Although we no longer fix the New Month by witnesses, but rather by a fixed calendar, in “full months” (months with thirty days; 5 months are always “full,” possessing 30 days, five always possess 29, and two fluctuate) both the 30th day of the last month, and the first of the new month, are Rosh Chodesh.
Another reason for this practice (see Shibolei Ha-Leket 168) is that the new moon is “born” on the thirtieth day of the month, and this is a reason for the special status of Rosh Chodesh applying even to the thirtieth day of the month.