Pesachim 110A

מצוה באפי נפשה הוא.

is a distinct mitzva in its own right. In other words, each cup is treated separately and one is not considered to be drinking in pairs.

״לא יעשה צרכיו תרי״, אמאי? נמלך הוא! אמר אביי, הכי קאמר: לא יאכל תרי וישתה תרי, ולא יעשה צרכיו אפילו פעם אחת ⁠— דילמא חליש ומיתרע.

The baraita taught that one should not attend to his sexual needs in pairs. The Talmud asks: Why should one be concerned for this; he has changed his mind? One does not plan in advance to engage in marital sex twice, and therefore the two acts should not combine to form a dangerous pair. Abaye said: This is what the tanna is saying, i.e., the baraita should be understood in the following manner: One should not eat in pairs nor drink in pairs, and if he does so he should not attend to his sexual needs right afterward even once, lest he is weakened by the act and will be harmed for having eaten or drunk in pairs.

תנו רבנן: שותה כפלים ⁠— דמו בראשו. אמר רב יהודה: אימתי ⁠— בזמן שלא ראה פני השוק, אבל ראה פני השוק ⁠— הרשות בידו. אמר רב אשי: חזינא ליה לרב חנניא בר ביבי, דאכל כסא הוה נפיק וחזי אפי שוקא.

A baraita states in another baraita: If one drinks in pairs his blood is upon his head, i.e., he bears responsibility for his own demise. Rav Yehuda said: When is that the case? When one did not leave the house and view the marketplace between cups. However, if he saw the marketplace after the first cup, he has permission to drink another cup without concern. Likewise, Rav Ashi said: I saw Rav Ḥananya bar Beivai follow this policy: Upon drinking each cup, he would leave the house and view the marketplace.

ולא אמרן אלא לצאת לדרך, אבל בביתו ⁠— לא. אמר רבי זירא: ולישן ⁠— כלצאת לדרך דמי. אמר רב פפא: ולצאת לבית הכסא ⁠— כלצאת לדרך דמי. ובביתו לא? והא רבא מני כשורי.

And we said that there is concern for the safety of one who drinks in pairs only when he intends to set out on the road after drinking, but if he intends to remain in his home there is no need for concern. R' Zeira said: And one who plans to sleep is comparable to one who is setting out on the road. He should be concerned that he might be harmed. Rav Pappa said: And going to the latrine is comparable to setting out on the road. The Talmud asks: And if one intends to remain in his home, is there no cause for concern? But Rava would count the beams of the house to keep track of the number of cups he had drunk so as to ensure that he would not consume an even number.