Answer (Rav Nachman bar Yitzchak): R. Yehudah taught the above Beraisa (Rav Chisda holds like Chachamim - whenever R. Yehudah says 'When is this?', this implies that he argues with Chachamim):

(Beraisa): If friends were reclining and uprooted themselves to go to synagogue or the Beis Medrash, they need not bless before leaving or when returning;

R. Yehudah: When is this? This is when some of them remained there - if not, they require a Berachah Acharonah on what they ate, and a new Berachah Rishonah when they return.

Inference: This is because they were eating foods that require a Berachah in their place (we infer this from 'uprooted', like above) - but regarding foods that do not require a Berachah in their place, even Chachamim mandate blessing before leaving and when returning!

Suggestion: This refutes R. Yochanan [who says that a change of wine or place does not mandate a new blessing]!

Question: He was already refuted above (101B)!

Answer: We suggest that also this Beraisa refutes him.

Rejection: No - he can say that Chachamim do not require blessings [when leaving and returning] even for foods that do not require a Berachah in their place;

It says 'uprooted' to teach the extremity of R. Yehudah's opinion - even regarding foods that require a Berachah in their place, if no one remained there, they must bless when they leave and return.

Support (for Rav Chisda - Beraisa): If friends were reclining to drink wine and uprooted themselves and returned, they need not bless (Rashi - it specifies wine because it requires a Berachah in its place).

(Beraisa - R. Yehudah): If friends were reclining together and Shabbos came, they bring a cup of wine and say Kidush on it [during the meal]; they say Birkas ha'Mazon on a second cup;

R. Yosi says, he may eat until (some say, even after) dark; when he finishes, he says Birkas ha'Mazon on the first cup and Kidush on the second.

Question: We should say both of them on one cup (Tosfos - according to R. Yosi, who says that both pertain to the Shabbos meal)!

Answer (Rav Huna): We do not say two Kedushos (Mitzvos (Rambam - mid'Oraisa)) on one cup.

Question: What is the reason?

Answer: We do not do Mitzvos in bundles [at once].

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 #1: The law is different when one does not have another cup. (This also explains why he may eat before Havdalah, i.e. in order that he will be able to say Birkas ha'Mazon over a cup.)

Question: When Yom Tov is on Motza'ei Shabbos, surely he has wine - nevertheless, Rav taught that he blesses [on one cup] on the wine, Kidush, light and Havdalah! (Normally, we are weak on Motza'ei Shabbos after losing the extra Neshamah of Shabbos, therefore we smell spices - this is not needed on Yom Tov, on which there is also an extra Neshamah; alternatively, the food and drink of Yom Tov suffice.)

Answer #1: Rav did not mention Zeman (Shehecheyanu) - surely, he discusses the last Yom Tov of Pesach [on which Zeman is not said - perhaps he is poor], and by then he has consumed his wine [except for one cup].

Objection: If the first Yom Tov is on Motza'ei Shabbos, everyone has wine [yet Kidush and Havdalah are said on the same cup]:

Abaye says that he blesses on the wine, Kidush, Zeman, light and Havdalah, and Rava says wine, Kidush, light, Havdalah and Zeman!

Answer #2 (to both questions): Rather, Kidush and Havdalah are similar (both discuss Kedushas Yom Tov, therefore they may be said on the same cup) - Birkas ha'Mazon and Kidush are unrelated.