In Diaspora, on the 1st day of Hol Hamo'ed Succot, which is the 3rd day of Succot, we read the Torah portion pertaining to the Musaph offering given on both the 2nd day as well as the 3rd day of Succot. This is because of Sfeika D'yoma. These 2 sections are recited also as part of the Musaph Shemoneh Esreh. The similar procedure is done on the 4th - 7th day of Succot - i.e. we recite the sections of the previous day as well as the current day.
Why aren't we consistent with this procedure on the 2nd day of Succot and recite the offerings for both the first and the 2nd day of Succot?