Two Mitzvot at Once: Ein Osin Mitzvot Chavilot Chavilot
The Talmud teaches that one must focus on each mitzva that one performs, and to give it one’s full attention. One is not permitted to try and perform two mitzvot at once. This principle is known as "ein osin mitzvot chavilot chavilot" (we do not perform mitzvot in bundles) in rabbinic literature. There is no concept of "killing two birds with one stone" when it comes to performing mitzvot.
One of the classic examples of ein osin mitzvot chavilot chavilot concerns the Sota procedure. A Sota is a woman who is suspected of committing adultery. In Temple times, such a woman was brought to the Beit Hamikdash in order to determine whether she was guilty or innocent. As part of a somewhat mystical procedure she was required to drink a special potion. If she was guilty of adultery, she died a horrible death on the spot. If she was innocent, she became the beneficiary of a number of different blessings. The Talmud rules that if two women appeared in the Beit Hamikdash at the same time, the Kohen was not permitted to perform the Sota procedure on both women simultaneously. Rather, he was required to administer the procedure on one woman at a time due to the rule of ein osin mitzvot chavilot chavilot.
Our sages forbade performing two mitzvot at once because doing so appears as if one is trying to discharge as many mitzvot as possible in the least amount of time. It also makes it appear as if performing mitzvot is a burdensome chore. So too, one is required to focus on one mitzva at a time without being distracted with the performance or details of a different mitzva. It is also noted that performing two mitzvot at once is at odds with the principle of "ha’osek b’mitzva patur min hamitzva" which teaches that one who is occupied with a certain mitzva is exempt from any other mitzva. In fact, performing a second mitzva while one is already involved with a different mitzva might actually disqualify it, as one was essentially "exempt" from having to perform it in the first place!
Furthermore, the intent to perform one mitzva inherently prevents the fulfillment of another. For example, one who forgot to recite Birkat Hatorah before shacharit may fulfill his obligation to do so with the ahava rabba blessing that is recited immediately prior to the shema. However, in order for the ahava rabba blessing to be an effective substitute for the Birkat Hatorah one is required to study some words of Torah immediately after praying. Although one might think otherwise, one’s recitation of shema does not count toward this required Torah study. This is because one’s intention when reciting shema is in order to fulfill the mitzva of kriat shema and not for the general mitzva of Torah study. One cannot discharge two different mitzvot in this single recitation. So too, one cannot fulfill the mitzva of shofar if one is blowing it both for the mitzva and in order to teach others how it is done. Here too, one is required to choose one “intention” when blowing the shofar: either to discharge the mitzva, or to teach others how it is done.
Further examples of ein osin mitzvot chavilot chavilot include the requirement to use two separate cups of wine at a wedding ceremony – one for the kiddushin and one for the nisuin. So too, one should not use the same cup of wine at a wedding meal for both the birkat hamazon and the recitation of the sheva brachot. Instead, a second cup of wine is poured for the sheva berachot. One should also not use the same cup of wine for both Kiddush and Birkat Hamazon. A kohen may not slaughter two animal sacrifices with a single stroke of a knife. Rather, each must be slaughtered individually. There are many other examples of ein osin mitzvot chavilot chavilot, as well. Nevertheless, in the era when polygamy was permitted, a man was permitted to marry several women at once. Similarly, it is permitted to celebrate a sheva berachot meal for more than one couple, even nowadays. One should not attempt to simultaneously discharge the mitzva of mishlo'ach manot and matanot la'evyonim by giving a poor person a very generous food package on Purim, though some authorities allow doing so.
According to most halachic authorities, it is not permitted to circumcise two babies, such as in a situation of twins, in a single ceremony due to ein osin mitzvot chavilot chavilot. Only after the brit of the first baby has been fully completed, along with all the accompanying prayers, may the brit of the next baby take place. In fact, two babies may not even be circumcised at the same time by two different mohels. There is a widespread custom in Israel to wait twenty minutes between each brit in order to demonstrate that the two ceremonies are separate, as well as to better justify the recitation of all the blessings anew. Others suggest that the main reason that the two ceremonies should be separate is due to ayin hara. It is interesting to note that the liturgy was arranged in such a way that no blessing ever contains two themes.
There is some discussion on how to proceed when two Torah scrolls must be removed from the aron kodesh for the Torah reading. There is an opinion that removing two (or more) Torah scrolls at the same time is problematic from the perspective of ein osin mitzvot chavilot chavilot. According to this approach, the congregation should remove only one Torah scroll from the aron kodesh at a time. After the first Torah portion is read, the Torah is returned to the aron kodesh, at which time the second Torah scroll is then removed for the additional reading. Nevertheless, common custom is not like this view, and multiple Torah scrolls are removed from the aron kodesh at the same time when they are needed for the day’s Torah readings.
One is permitted to eulogize two or more people at once should circumstances warrant doing so. This is because there is little concern that those present will suspect that one is combining the eulogies for the sake of convenience, or the like. In fact, eulogizing several people at once might even enhance the mitzva and show even greater honor to the deceased. This is because more people are likely to attend a eulogy for two people than for one person. This is especially true if the eulogies are for Torah scholars or otherwise outstanding individuals. There is also a view that the principle of "ein osin mitzvot chavilot" does not apply to mitzvot that are rabbinical in nature, as is the mitzva of eulogizing the deceased. Similarly, one is sometimes permitted to fast on a single day to fulfil two different fast-related observances or obligations.
Rav Ovadia Yosef rules that one may combine the Purim seuda with a siyum -- the celebration upon completing a tractate of Talmud. He rules that there is no concern that one would violate "ein osin mitzvot chavilot chavilot" even though the two events are celebrated in a single meal. He argues that the rule of "ein osin mitzvot chavilot chavilot" only forbids using one object to perform two separate mitzvot, such as using a single cup of wine for both Kiddush and Birkat Hamazon. One is permitted, however, to perform two mitzvot in a single act or event, such as combining a holiday meal and a siyum meal in one. Indeed, when the first night of Sukkot or Pesach is on a Friday night, one essentially discharges two mitzvot at once the moment that one partakes of bread or challa: the mitzva to eat bread in the Sukka (or matza on Pesach) and the mitzva to eat the Shabbat meal. There is also a view that the principle of "ein osin mitzvot chavilot chavilot" applies only to mitzvot that are both obligatory in nature. Celebrating a siyum, however, is optional in nature.
Closely related to the principle of ein osin mitzvot chavilot chavilot is the principle of "ein ma'arvin simcha b'simcha," that one may not combine two festive occasions or celebrations. For example, when Yaakov became aware that he had been tricked into marrying Leah instead of Rachel, he was forced to wait until the conclusion of the sheva brachot week before he could marry Rachel. This is because marriage celebrations continue for a full week. If one would marry another woman during this time, one would essentially be celebrating two marriages at once, a violation of "ein ma'arvin simcha b'simcha."
It is also forbidden to hold a wedding on Chol Hamoed due to ein ma'arvin simcha b'simcha. This is because on Chol Hamoed we are required to focus exclusively on celebrating the holiday and not on celebrating anything else. Some authorities even rule that a pidyon haben that is scheduled to take place on Chol Ha’moed must be postponed until after the holiday due to ein ma'arvin simcha b'simcha.
 Tosfot, Moed Katan 8b; Yerushalmi Moed Katan 1:7.
 Sota 8a.
 Rashi, Sota 8a; Rashbam, Pesachim 102b.
 Berachot 49a; Pesachim 102b; Tosfot, Moed Katan 8b.
 Rosh Hashana 29a; Maharshag 4:29.
 Ran, Rosh Hashana 32b.
 Beit Yosef, OC 47.
 B’tzel Hachachma 1:1. Note: There are grounds to suggest that one may fulfill two mitzvot with a single action if one has explicit intention to do so.
 Pesachim 102b; Machzor Vitri 469; EH 62:9; Rema, EH 65:3; Shulchan Ha'ezer 8:3:6; Yabia Omer, EH 9:23.
 Pesachim 102b.
 Shabbat 117b; Sota 8a; Pesachim 102b.
 Rambam, Hilchot Ishut 10:14; EH 62:2.
 See Emek Halacha 2:43 for more on this.
 Tosfot, Sota 8b; Magen Avraham 147:11.
 For more on a double brit ceremony see: Beit Shmuel, EH 62:3; Shach, YD 265:25; Taz, YD 265:11; Pitchei Teshuva, YD 265:10; Tashbetz 2:42; Maharam Schik, YD 250.
 Berachot 49a.
Yerushalmi Megilla 4:5; Magen Avraham 147:11.
 Divrei Malkiel, YD 1:69.
 Ketav Sofer, OC 39.
 See OC 568:11 with commentaries.
Chazon Ovadia, Laws of Purim p. 181-185, cited at: http://www.dailyhalacha.com/Display.asp?PageIndex=9&ClipID=1391.
 The widespread custom to use the same cup of wine for both birkat hamazon at seudat shlishit and for havdalla is indeed problematic. Nevertheless, some justify this practice arguing that it is permitted to perform a Torah mitzva (birkat hamazon) and a rabbinic mitzva (havdalla) on a single item. See for example: Meiri, Shabbat 102b; Biur Halacha 299.
 Magen Avraham 147:11.
Tosfot, MoedKatan 8b.
 Tosfot, Moed Katan 8b.
 YD 305:11; Shach, YD 305:13. See Otzar Pidyon Haben 17:6 who permits a pidyon haben on Chol Ha’moed and common custom is in accordance with this view.