Chabad.org — English translation + Rashi
Torah Temimah (Pesachim 93a): the kares applies to one who omitted the first Pesach willfully; opinions differ on the relationship to the second.
English: The Sages taught: one is liable to kares for the first and liable to kares for the second — the words of Rabbi. Rabbi Noson says: liable to kares for the first, and exempt for the second. Rabbi Chananya ben Akavya says: even for the first he is not liable to kares unless he did not offer the second.
Relationship of the two months — the second is a remedy for those who were unable, not a free pass for those who were able and refused.
Willful neglect of the first remains a distinct violation; the Tannaim debate how the second interacts with that liability.