Keritot, Chapter Five, Mishnah Eight
The final mishnah of our chapter deals with a case where one of the pieces of meat was regular forbidden fat and the other was forbidden fat and notar (remnant).
1) If there was a piece of forbidden fat and another piece of forbidden fat [which was at the same time] notar, and a person ate one of them and does not know which, he is liable to a hatat and to an asham talui.
2) If he then ate the second piece, he is liable to three hatats.
3) If he ate one piece and another came and ate the other, each of them brings a hatat and an asham talui.
a) Rabbi Shimon says: each of them brings a hatat and together they bring another hatat.
b) Rabbi Yose says: any hatat that is brought for the expiation of sin cannot be offered by two people.
Section one: If he eats one of the pieces, he certainly must bring one hatat for eating helev. He must also bring an asham talui lest he ate the piece that is also notar.
Section two: Two hatats are for the forbidden fat, assuming that they were eaten in two periods of unawareness and the third hatat is for eating notar.
Section three: They both must bring one hatat for they both certainly ate helev. According to the first opinion they both bring an asham talui, for each one of them might have also eaten helev. Rabbi Shimon holds they can jointly bring a second hatat. And Rabbi Yose again holds that two people can never jointly bring a sacrifice that comes to expiate for sin.