As we have already learned (see Pesahim 100), Rabbi Yehuda and Rabbi Yose differ on how to deal with a situation where a Friday afternoon meal continues into Shabbat. According to Rabbi Yehuda you must end your meal in order to stop and welcome Shabbat; Rabbi Yose rules that you can continue your meal.
The discussion on our daf relates to the cups of wine that must be drunk to close the meal and to welcome the Shabbat. According to the baraita we will need two separate cups of wine, a ruling explained by Rav Nahman bar Yitzhak as stemming from the principle ein osin mitzvot havilot havilot – that we do not perform mitzvot “in bundles.” The idea is that every mitzva deserves its own focus, and if we try to perform several mitzvot with the same cup of wine it will be impossible to focus on each mitzva separately. A similar idea is ein me’arvin simha be-simha – that we do not combine two joyous occasions (e.g. to have a wedding during Pesah or Sukkot), because each one deserves its own focus.
The Gemara distinguishes between a case where we want to combine Kiddush together with birkat ha-Mazon (Grace after meals), when this rule would apply, and a case where we need to combine Kiddush and havdala (the separation service after Shabbat or Yom Tov), like when one of the holidays (Pesah, Shavu’ot or Sukkot) falls out on Saturday night and we need to make havdala to commemorate the end of Shabbat and Kiddush to usher in the holiday. In such a case, the Gemara rules that it would be appropriate to use one cup for both ceremonies, since Kiddush ve-havdala hada milta he – the ceremonies of Kiddush and havdala are one and the same, while Kiddush and birkat ha-Mazon are two different things.
In explanation of this statement, some rishonim argue that Kiddush and havdala are similar in that they introduce a meal, while birkat ha-Mazon ends the meal. Others point out that havdala contains an aspect of Kiddush in that it serves to emphasize the uniqueness of Shabbat in distinguishing between Shabbat and the weekday. Another suggestion that is raised is that, in this case, Kiddush and havdala are actually dependent on one another, since the holiday cannot begin until Shabbat ends. When we announce that Shabbat is over, we effectively welcome the holiday; when we welcome the holiday we are calling for an end to Shabbat. Birkat ha-mazon has no such relationship with Kiddush at all.