Gemara

(Beraisa #1 - R. Yosi): If Shimon had Bi'ah with Rachel (before Leah died), he is liable for the Isurim of Eshes Ach and Achos Ishto;

R. Shimon says, he is liable only for Eshes Ach.

Contradiction (Beraisa #2 - R. Shimon): He is liable only for Achos Ishto!

Answer: (R. Shimon always holds that only the first Isur takes effect.) In Beraisa #2 Shimon married (Leah) before Reuven married (Rachel, so the Isur of Eshes Ach took effect first). In Beraisa #1, Reuven married first.

Question: According to R. Shimon, when Reuven married first, since the Isur of Achos Ishto does not take effect, Yibum should be permitted!

Answer (Rav Ashi): The Isur is pending. If the Isur of Eshes Ach would go away, the Isur of Achos Ishto would take effect. Therefore, Yibum is forbidden, and the Isur of Eshes Ach remains.

Question: Elsewhere, R. Yosi holds that Ein Isur Chal Al Isur!

(Beraisa): If a man did an Aveirah punishable by two different deaths, he receives the more severe one;

R. Yosi says, he is sentenced to the first that applied.

(Beraisa): (R. Yosi holds that) if Chamoso (his mother-in-law) got married, he is liable for her for Chamoso. If she was married and then became his mother-in-law, he is liable for Eshes Ish.

Answer #1 (R. Avahu): R. Yosi agrees that that Isur Chal Al Isur if the latter Isur is Mosif (it forbids the forbidden object to more people).

Objection: This explains the case when Shimon married before Reuven did. When Rachel married Reuven, she became forbidden to all his brothers, so the Isur of Eshes Ach also takes effect on Shimon (her sister's husband).

But when Reuven married first, why is Achos Ishto an Isur Mosif (when Shimon marries Leah? Rachel does not become forbidden to more people!)

Suggestion: It is Mosif because Shimon became forbidden to all the sisters.

Rejection: This is an Isur Kolel (it forbids more objects on the person)!

Answer #2 (Rava): Really, he is liable only for one. R. Yosi means that he is considered liable for both. This teaches that he is buried among the utterly wicked (like one who did two Aveiros).

R. Chiya and Bar Kapara argue about a Zar (non-Kohen) who served in the Mikdash on Shabbos, like R. Yosi and R. Shimon argue.

R. Chiya: Shabbos was forbidden to all. It was permitted (for Avodah) only to Kohanim. He is liable for transgressing Shabbos, and for Avodas Zar.

Bar Kapara: Shabbos was forbidden to all. It was permitted (Stam, i.e. to all). He is liable only for Avodas Zar.

Similarly, if a Kohen Ba'al Mum served in the Mikdash b'Tum'ah, Bar Kapara is Mechayev only once, for Avodas Ba'al Mum. R. Chiya is also Mechayev for Avodah b'Tum'ah. Similarly, if a Zar ate Melikah, Bar Kapara is Mechayev only for Achilas Kodshei Kodoshim by a Zar, and R. Chiya is Mechayev also for eating Neveilah. In each case R. Chiya swore that Rebbi said like himself, and Bar Kapara swore that Rebbi said like himself.

Question: What do they argue about?

Answer: They argue about Isur Bas Achas (Isurim which come simultaneously), according to R. Shimon. R. Chiya holds that R. Shimon is Mechayev twice; Bar Kapara holds that he is Mechayev once.

Question: How can the first two cases be Isur Bas Achas?

Answer: Regarding a Zar who served on Shabbos, the case is that he brought two hairs (after 13 years, and became an adult) on Shabbos. (All Isurim, including) Avodas Zar and Melachah on Shabbos take effect on him when he becomes an adult.

Likewise, if a Kohen Ba'al Mum was Tamei when he became an adult, the Isurim of Avodas Ba'al Mum and Avodah b'Tum'ah both come the moment he becomes an adult. Alternatively, his finger was cut off with a Tamei knife, making him Tamei and blemished at the same time.

33b - Question (Beraisa - R. Yosi): If a Zar served on Shabbos, and a Ba'al Mum Kohen served when Tamei, the former is liable for Avodas Zar and Melachah on Shabbos; the latter is liable for Avodas Ba'al Mum and Avodah b'Tum'ah;

R. Shimon says, the former is liable only for Avodas Zar; the latter is liable only for Avodas Ba'al Mum.

The Beraisa omitted the case of Melikah. This is not due to R. Yosi (because it is inconsistent with his other teachings). He is Mechayev twice for Isur Kolel, all the more so for Isur Bas Achas! Rather, it was omitted due to R. Shimon. He is Mechayev only once for Isur Kolel, but agrees that one is liable twice for Bas Achas.

Bar Kapara is refuted.

Rishonim

Question: Why was the Gemara (32b) sure that R. Yosi holds that Ein Isur Kolel Chal Al Isur?

Answer (Tosfos Yevamos 32b DH Isur): The Gemara held that when an Eshes Ish became Reuven's Chamoso this is Kolel, because other relatives of Reuven's wife become forbidden to him. Nevertheless R. Yosi is Mechayev only the Misah for Eshes Ish. Later (33a) the Gemara says that R. Yosi holds that Isur Kolel is Chal Al Isur. It holds that an Eshes Ish who became Chamoso is not Kolel, because the other relatives are not forbidden by the same Shem (Isur). A Ba'al Mum who served b'Tum'ah is called Kolel because Tum'ah forbids eating Kodshim, and this is also an Isur of 'Kereivah' - "Ish Asher Yikrav..." Regarding Isur Mosif, it does not matter whether or not it is the same Shem.

Question (Chachmas Shlomo 33a DH Gemara): Why did the Gemara need to say that his finger was cut with a Tamei knife? We could say that first he was Tamei, for this is Kosher for Avodas Tzibur. When he becomes a Ba'al Mum the Isurim come together!

Answer #1 (Chachmas Shlomo): The Gemara holds that Tum'ah is Dechuyah b'Tzibur, not Hutrah (it is permitted only if we cannot do the Avodah b'Taharah). Even though his Avodah is Kosher b'Di'eved in any case, since he may not serve l'Chatchilah, the Isur Tum'ah preceded the Isur Ba'al Mum.

Answer #2 (Maharsha DH Gemara): Since he was already Tamei and was nevertheless Kosher for Avodas Tzibur, when he becomes a Ba'al Mum the Isur Tum'ah would not take effect.

Rambam (Hilchos Isurei Bi'ah 17:8): A great Kelal regarding all Isurim is that Isur Chal Al Isur only if they come together, or if the latter is Mosif other things on the first Isur, or it is Kolel other things together with the latter Isur.

Rambam (Hilchos Bi'as Makdish 9:12): If a Zar served in the Mikdash on Shabbos, he is liable for Chilul Shabbos and for Avodas Zar. Similarly, if a Kohen Ba'al Mum served in the Mikdash b'Tum'ah, he is liable for Avodas Ba'al Mum and for Avodah b'Tum'ah.

Ra'avad: This is when they came b'Vas Achas, e.g. he brought two hairs on Shabbos, or he became a Ba'al Mum when his finger was cut off with a Tamei knife.

Question (Kesef Mishneh): Why does the Ra'avad require Bas Achas? The Gemara said that he is liable twice even if they were not b'Vas Achas, due to Kolel!

Mishneh l'Melech: The Toras Kohanim says that the Isur of Shtuyei Yayin applies to "Atah u'Vanecha", but not to a Chalal or Ba'al Mum.

Question (Tosfos 32b DH Isur): According to the version that R. Yosi agrees that Isur Chal Al Isur when it is Kolel, why isn't Chamoso Chal on Eshes Ish?

Answer (Tosfos): It is not considered Kolel because the other matters that become forbidden together with Chamoso (his wife's other relatives) are forbidden due to different Isurim than Chamoso.

Question: Also when a Ba'al Mum became Tamei, the other matter forbidden (eating) is a different Isur (than Avodah)!

Answer #1 (Tosfos): Eating and Avodah are considered the same Isur, for both are called 'Kereivah'.

Answer #2 (Aruch l'Ner 33a DH Iy Nami citing Rashba): When the Gemara said that Tum'ah is Chal due to Kolel, it could have asked that if Kolel applies even to also Chamoso is Kolel! Rather, it asked a better question. Alternatively, the conclusion is that they are considered Kolel only for burial among Resha'im who transgressed two Aveiros.

Answer (to Question 4:ii - Aruch l'Ner): The Rambam holds like Tosfos. The Ra'avad answers Tosfos' latter question like the Rashba.