The Talmud teaches the principle that a melakhah that is normally done by one person does not incur liability when performed by two people together (shnayim sh’asauhu). It would seem that there are at least two ways to understand this concept: as an indication that the melakhah itself is a deficient, and thus ineligible for punishment; alternatively, it may be that the two people performing the action are both unable to be blamed for an action that can equally be assigned to the other.

Many issues may relate to this question, including the source for the principle, which is open to some discussion: it may be a logical principle, similar to the “lo k’derekh” (unusual) exemption found by many areas of Biblical law (note the language of Rashi, 92b); or it may be derived from the verse (Vayikra 4:27), “b’asotah” (by committing [singular]); or it may be a combination of the two.

Related to this question as well would be the scope of the exemption: it is purely an exemption from the required sacrifices for inadvertent violations, or is it completely permitted (at least on a biblical level). Similarly, R. Akiva Eiger (hagahot to Rambam, Hil. Shabbat 1:15) questions whether the exemption extends to Yom Tov, which carries a punishment of malkot. He notes that the Rambam (Hil. Kilayim 9:9) rules there is no exemption for the act of plowing with a forbidden mixture of animals, even when the animals are driven by several people (note, though, Resp. L’Horot Natan, XII, 14). R. Yosef Engel (Resp. Ben Porat, 10) maintains that the exemption is only for the sacrifice but the prohibition remains. (In regard to other prohibitions, see also Bekhor Shor, Chulin 82a [oto v’et b’no]; Ritva, Kiddushin 43a [sh’chutei chutz]; Tosafot, Avodah Zarah 60a, s.v. havah [yayin nesekh]; Mishmeret Chaim [basar b’chalav]; Minchat Chinukh, 15 [hotza’at haPesach bachutz; see also Resp. Rashba, I, 25], 26 [shechitah l’avodah zarah], and 144 [pigul]; Resp. Avnei Nezer. O.C. 51:7 and Y.D. 393:10; Resp. Chavot Yair, 112; Resp. Noda B’Yehudah, II, O.C. 76; and Resp. Ben Porat, ibid.).

Another issue considered is whether the exemption applies if the second individual is one not subject to the prohibition, a question posed by the Maharit Algazi (Simchat Yom Tov, 59). As the Resp. Yad Sofer (#60) observes, if the exemption is based on the action being done in an “unusual” manner, it would not matter that the activity is being shared with someone not part of the prohibition.

R. Herschel Schachter (Eretz HaTzvi, 4, n. 5) notes that in that the verse that would serve as the source for this concept is not referring to Shabbat but rather to the korban chatat, if it is to applied to Shabbat it must be to teach that the action itself is fundamentally flawed; that being the case, it would fail to meet the “melekhet machshevet” clause for Shabbat. Consequently, the exemption would be absolute, applicable to Yom Tov as well, and indicate that the activity is actually permissible (biblically). In addition, the extent to which it is rabbinically prohibited is also affected by this understanding. If the action is indeed fundamentally flawed, it must be then that it is not a “rabbinical melakhah”, which would be subject to several stringencies, but a protective g’zeirah.