Pesachim 102B

גמרו, כוס ראשון מברך עליו ברכת המזון והשני אומר עליו קדושת היום. אמאי? ונימרינהו לתרוייהו אחדא כסא!

R' Yosei maintains that once they have finished their meal, they bring out two cups; over the first cup one recites the Grace after Meals, and over the second cup he recites the sanctification of the day. The Talmud asks: Why do they need two cups? And let them say both of them, Grace after Meals and kiddush, over one cup.

אמר רב הונא אמר רב ששת: אין אומרים שתי קדושות על כוס אחד. מאי טעמא? אמר רב נחמן בר יצחק: לפי שאין עושין מצות חבילות חבילות.

Rav Huna said that Rav Sheshet said: One does not recite two sanctifications, i.e., for two mitzvot such as Grace after Meals and kiddush, over one cup. What is the reason for this halakha? Rav Naḥman bar Yitzḥak said: Because one does not perform mitzvot in bundles. If someone performs multiple mitzvot all in one go, he gives the impression that they are a burdensome obligation that he wants to complete as fast as possible.

ולא? והא תניא: הנכנס לביתו במוצאי שבת מברך על היין ועל המאור ועל הבשמים, ואחר כך אומר הבדלה על הכוס. ואם אין לו אלא כוס אחד ⁠— מניחו לאחר המזון ומשלשלן כולן לאחריו. אין לו שאני.

And does one not perform multiple mitzvot together? But wasn’t it taught in a baraita: One who enters his home at the conclusion of Shabbat recites the blessing over the wine, and then over the light, and then over the spices, and after that he recites havdala over the cup of wine. And if he has only one cup of wine, he leaves it for after he eats his food, and uses it for Grace after Meals, and arranges all of the other blessings together after that. This baraita indicates that one may use the same cup both for Grace after Meals and havdala. The Talmud answers: We cannot prove anything from here, as a case where one does not have an additional cup is different. One who has two cups of wine is required to recite Grace after Meals over one of the cups and havdala over the other one.