Tashlich is performed just once over the two days of Rosh Hashanah, usually on the first day (unless that's Shabbos)
If one had a personal ritual / event one liked to perform / hold each month to mark Rosh Chodesh then, in those instances when Rosh Chodesh lasts two days, the 30th of the old month and the 1st of the new month, would it be better to perform / hold it on the first or the second day? (Supplementary question: if practical, would it be ideal to perform it on both days?)