Keritot, Chapter Five, Mishnah Five
Todays mishnah is virtually the same as yesterdays mishnah, except that in todays mishnah a person is not sure whether he ate permitted unconsecrated fat, or helev, for which one brings a hatat. In other words, yesterday the question was whether one had to bring an asham, today the question is whether one needs to bring a hatat.
Since the mishnah is so similar to yesterdays mishnah, my comments below are quite brief.
1) If there was a piece of forbidden fat and a piece of hullin [permitted fat], and a person ate one of them and does not know which, he is liable to an asham talui.
2) If he then ate the second piece, he is brings a hatat.
3) If he ate the one [piece] and another came and ate the other, each of them is liable to an asham talui.
a) Rabbi Shimon says: they together bring one hatat.
b) Rabbi Yose says: two people cannot bring one hatat.
Section one: Since he doesnt know if he sinned, he brings an asham talui.
Section two: Since he knows that he did sin, he brings a hatat, the sacrifice for accidentally eating helev, forbidden fat.
Section three: According to the first opinion, since we dont know which of the two ate helev, they both bring an asham talui. This is not any different, at least from the perspective of the individual, from the first case.
Rabbi Shimon says that the two people can bring a hatat jointly and stipulate that if the first one ate the helev, that it is his hatat, and if the second one did, it is his hatat.
Rabbi Yose says that people cannot jointly bring a hatat.