Gemara

R. Shimon explains that "Osah" teaches that a Sotah drinks alone, lest she encourage another Sotah to drink. If she is terrified, she may drink with another.)

Question (Beraisa): We do not give two Sotos to drink at once. We are not Metaher two Metzora'im at once... (because) we do not do Mitzvos in bundles.

Answer (Abaye): We distinguish between one Kohen and two.

Pesachim 102a (Beraisa - R. Yosi): If friends were eating together and Shabbos came, they may eat until dark. When they finish, they say Birkas ha'Mazon on one cup and Kidush on a second.

Question: We should say both of them on one cup!

Answer (Rav Huna): We do not say two Kedushos on one cup.

(Rav Nachman): This is because we do not do Mitzvos in bundles.

Question (Beraisa): On Motza'ei Shabbos one blesses on wine, light, and spices; then he says Havdalah on (i.e. before drinking) the wine. If he has only one cup, he (waits until after he eats, and) says all of them after Birkas ha'Mazon.

Answer: The law is different when one does not have another cup.

Question: When Yom Tov is on Motza'ei Shabbos, Rav, Abaye and Rava hold that one he blesses (on one cup) for wine, Kidush, light and Havdalah!

Answer: Kidush and Havdalah are similar (they may be said on the same cup, for both discuss Kedushas Yom Tov). Birkas ha'Mazon and Kidush are unrelated.

Rishonim

Rif and Rosh (Pesachim 20b and 10:8): If friends were eating and Shabbos came, they say Birkas ha'Mazon on one cup and Kidush on a second. We do not say two Kedushos on one cup, for we do not do Mitzvos in bundles. When Yom Tov is on Motza'ei Shabbos, we bless Kidush and Havdalah on one cup, for they are similar. Birkas ha'Mazon and Kidush are unrelated.

Ran (DH Kos): Kidush and Havdalah are both for the entrance of the new day. Birkas ha'Mazon is for the past, unlike Kidush, which is for what enters.

Rosh: This is why we do not say six Berachos on the cup of Birkas ha'Mazon at a Chupah. Rather, we bring another cup, say six Berachos over it, and say Borei Peri ha'Gafen over the cup of Birkas ha'Mazon. This is not astounding, to make six Berachos over a cup and not say Borei Peri ha'Gafen on it. Rav Ashi blessed only on the first cup he was served. R. Meshulam enacted to say the Berachos of Kidushin and Nisu'in over one cup. They are unlike Birkas ha'Mazon and Kidush, which are different: one is after the meal, and one sanctifies the day. R. Tam requires two cups, for we say Birkas Kidushin in Beis Eirusin. Also, sometimes Nisu'in is long after Kidushin, and in some places they read the Kesuvah in between Kidushin and Nisu'in.

Tosfos (102b DH she'Ein): We intend to interrupt between the Berachos of Kidushin and Nisu'in, therefore they require two cups.

Tosfos (102b DH she'Ein): Because we do not say two Kedushos over one cup, some do not say Sheva Berachos over the cup of Birkas ha'Mazon at a Chupah. Rather, we bring another cup, and do not say Borei Peri ha'Gafen on it, for we already blessed on the cup of Birkas ha'Mazon. It is improper to bless twice. R. Meshulam blessed on the same cup, for here they are one matter. Birkas ha'Mazon causes that we say Sheva Berachos.

Rambam (Hilchos Sotah 4:2): We do not give two Sotos to drink at once, for it says "v'He'emid Osah."

Poskim

Shulchan Aruch (EH 62:9): Some say not to say Sheva Berachos on one cup. Rather, we bring another cup, say Sheva Berachos over it, and say Borei Peri ha'Gafen over the cup of Birkas ha'Mazon. Some do not require this. Rather, we say the Sheva Berachos over the cup of Birkas ha'Mazon. This is the custom.

Magen Avraham (147:11 DH uv'Sotah): Tosfos forbids one Kohen to bring two Sotos to drink at once, but two Kohanim may give Sotos to drink at once. The Rambam permits with one Kohen, for he cannot give them to drink at once, but two Kohanim might make them drink at once. Tosfos forbids bringing two cups at once. After Birkas ha'Mazon, we bring another cup for Sheva Berachos. It seems that the Tur and Shulchan Aruch agree.

Drishah (18): We say "Devai Haser..." (before Birkas ha'Mazon) on the cup of Birkas ha'Mazon. Since it does not have a Berachah, it is not a problem of Mitzvos in bundles. The Tur says 'after Birkas ha'Mazon, they bring another cup...' This shows that one need not prepare the second cup beforehand.

Rebuttal (Taz 7): We say "Devai Haser..." only when we will be saying Sheva Berachos, therefore it is proper to say it on the cup of Sheva Berachos. After "Devai Haser", we pick up the cup of Birkas ha'Mazon.

Rema: In our lands the custom is like the first opinion.

Ha'Makneh (Kuntres Acharon, cited in Pischei Teshuvah 17): Even if Yakov did not say Birkas ha'Mazon, he can say the Sheva Berachos, and the one who said Birkas ha'Mazon blesses Borei Peri ha'Gafen and is Motzi Yakov on Yakov's cup. The one who said Birkas ha'Mazon blesses on incense (Berachos 42b). Birkas Nisu'in is different; it is a separate matter. The Rambam (Hilchos Berachos 2:9) and Shulchan Aruch (Sa'if 5) agree; they do not allow a slave or minor to say Sheva Berachos. Surely a slave or minor did not say Birkas ha'Mazon, for he cannot be Motzi others! We find that Levi came to Rebbi's house and blessed six Berachos! (Kesuvos 8a says that Rav Asi blessed six, and Levi blessed only five, for he blessed only one Berachah of Yetzirah). We must say that someone else blessed Birkas ha'Mazon!

Pischei Teshuvah (18): The Ya'avetz permits saying Sheva Berachos on the cup of Birkas ha'Mazon on Seder night, unlike the custom. Since one may not add to the four cups, we rely on the latter opinion. If he wants to use two cups, he saves it for his fourth cup. Chaim She'al (74) prefers to bless Sheva Berachos on cup of Birkas ha'Mazon. We should follow the Darchei Moshe (473:5), who says that we bless Sheva Berachos on the Chasan's cup.

Aruch ha'Shulchan (18): Our custom is that the one who blesses drinks most of the cup, mixes the two cups and gives to the Chasan and Kalah to drink.

Rema (ibid.): Some say that even Asher Bora itself requires two cups.

Bach (DH v'Yesh Omrim): It is proper to do so.

Shulchan Aruch (ibid.): We say the Berachos of Kidushin and Nisu'in over two cups, even if Kidushin is at the time of Chupah.

Chelkas Mechokek (15): We bless Borei Peri ha'Gafen on each, but one Berachah Acharonah for both.

Rema: The custom is to interrupt in between with reading the Kesuvah.