The Gemara asks why they were mesakein 4 Kosos of wine, if they are Zugos.
Insights http://www.dafyomi.co.il/pesachim/insites/ps-dt-109.htm #4 discuss the question of Lechem Mishneh, which are two loaves of Lechem.
Shas Lublin brings that the Tzlach asked why the Gemara s question focused on the 4 Kosos? Every Shabbos we have Kiddush and the Kos of Bircas HaMazon. (They add especially according to the position that every individual needs a Kos. By adding this Kos it becomes Zugos.)
Without having gone through the Gemara, there may be an answer on this. Even if we say that the Kos of Kiddush and the Kos of bentching are equal chiyuvim, they have nothing to do with each other. They are two separate Halochos, two separate mitzvohs that happen to be at the same seuda. That may not be Zugos. Zugos may only be like the 4 Kosos by the Seder, where there was a Takana of FOUR. Four Kosos. Or like Lechem Mishneh which is a Halacha of two. Zugos may also be where the exact same thing is repeated twice for the same reason. Eating twice, wiping twice. He is eating TWO times, or DOUBLE. The Kosos Friday night, are not drank to be DOUBLE. They are not a takana of two Kosos. They are just separate mitzvohs that come together. Perhaps that is not Zugos.
M. Reinitz
Dear Moshe,
Actually, the third answer of the Gemara by Ravina says that even the 4 Kosos are separate Mitzvos and therefore are not Zugos. This would answer the Tzelach's question.
The Tzelach himself says there's no reason to count them as two since Birchas haMazon causes a break. (He further explains that the main question of 4 Kosos is either the first pair or the second pair of the 4 Kosos).
All the best,
Reuven Weiner