OT1H - On YT Sheini of Pesach we begin S'firat ho'Omer and we also recite those Maaroviyot that deal with K'tzirat ho'Omer which took place on the 16th of Nissan - See EG Kitzur SA 120:5 for some details.
OTOH - regarding YT Sheini of Sukkot we are told NOT to mention the Korban of the 2nd day in Mussaf that should be mentioned due to s'feika d'yoma - because that would be a zilzul of YT Sheini d'Galuyot.
See O"Ch Ba'er Hetev 663:2 quoting a Rashi in Megillah 30
Also Mishnah Brurah 663:3
Sha'ar haTziyun 2
And so - how come that reciting the Maaraviyot of Ketzirat ho'Omer during YT Sheini of Pesach does not pose the same issue of zilzul YT as does reciting the korban "Uvayom Hasheini" on YT Sheini of Sukkot?