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It is rare but possible for conjoined twins to remain alive and healthy into adulthood, such as is the case with Abby and Brittany Hensel. They have distinct sets of heads, hearts, and lungs. They have two arms and legs and a shared reproductive system. One can be asleep while the other is awake. They have distinct personalities, desires, and opinions.

Suppose there were a set of Jewish conjoined twins in similar circumstances, either male or female. My basic question is this: In terms of halachah, would they be considered one person or two?

There are a range of issues to which this would apply. For example:

  • Would they (as men or women) be permitted to marry, and if so, would they marry one person or two, with one ketubah or two?
  • As men, would they count as one or two for a minyan or zimmun?

What are some other issues that arise from this situation?

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Maybe it matters where they are cojoined. – Daniel Sep 2 '13 at 21:26
    
Yes, that's part of what I'm wondering. – Cislunar Sep 2 '13 at 21:40
    
There are probably 1000 other cases you could ask this about. Interesting that you chose these two. (BTW minyan and zimmun are not necessarily parallel here.) – Double AA Sep 2 '13 at 22:11
    
I had lots of ideas, but I wanted to keep the question as focused as possible. I thought the question of "how many people are they" was the root issue and that these issues exemplified the problem. – Cislunar Sep 2 '13 at 22:21
1  
@Annelise If the twins are considered two separate people, I don't see how marriage would be possible: Besides for the prohibition of marrying one's wife's sister in her lifetime, yichud would be impossible. – Nᴮᶻ Sep 9 '13 at 14:48

.מנחות דף לז

בעא מיניה פלימו מרבי מי שיש לו שני ראשים באיזה מהן מניח תפילין א"ל או קום גלי או קבל עלך שמתא אדהכי אתא ההוא גברא א"ל איתיליד לי ינוקא דאית ליה תרי רישי כמה בעינן למיתב לכהן אתא ההוא סבא תנא ליה חייב ליתן לו י' סלעים איני והתני רמי בר חמא מתוך שנאמר פדה תפדה את בכור האדם שומע אני אפילו נטרף בתוך ל' ת"ל אך חלק שאני הכא דבגולגולת תלא רחמנא

Soncino translation:

Pelemo enquired of Rabbi, If a man has two heads on which one must he put the tefillin?’ ‘You must either leave’, he replied, ‘or regard yourself under the ban’. In the meantime there came a man [to the school] saying, ‘I have begotten a first-born child with two heads, how much must I give the priest?’ An old man came forward and ruled that he must give [the priest] ten sela's. But this is not so! For Rami b. Hama learnt: From the verse. The firstborn of man thou shalt surely redeem, I might conclude that this would apply even when the firstborn was rendered trefah within thirty days [of his birth]. Scripture therefore added, Howbeit, limiting thereby [the general application]! — In this case it is different since the Divine Law declared [the law of redemption] to be governed by the expression ‘per head’.

It would appear that for הלכות that we count specifically the head, siamese twins count as two, but for anything else they count as one.

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How do you get "but for anything else they count as one"? Anyway it's odd, since one of the heads would have emerged first and he'd be firstborn, right? – Annelise Sep 3 '13 at 13:03
    
it says ''In this case it is different since the Divine Law declared [the law of redemption] to be governed by the expression ‘per head'', leading me to the conclusion that this דין is applicable specifically where there is a גזירת הכתוב, no where else. – moses Sep 3 '13 at 18:14
    
it also appears that in order for this פסק to work you would need to say אפשר לצמצם, that they can indeed come out as one. if you held אי אפשר לצמצם, the father would probably be חייב only one פדיון הבן. – moses Sep 3 '13 at 18:15

There are a number of halachic issues, for example, the Gemarah in Menachot (page 37) discusses conjoined twins with regard to tefillin, and Piskei Teshuvot 225:20 in name of Sh”t Shevut Yacov 1:4 writes that one may make the blessing of "Meshaneh HaBriyot" on Siamese twins.

The most famous/talked about is the ethical/halachic dilemma of separating twins where doing so can/will do harm (to ''one' of them').

Here's a list of resources on this topic:

The Conjoined Twins Dilemma: Shnayim Mehalchin B’derech, T’nu Lanu Echad Mi’khem & Rodef

"Jewish Medical Ethics: Siamese Twins" An Unpublished Responsum by Rav Moshe Feinstein zt"l Translated and annotated by Rabbi Moshe Dovid Tendler (backup link)

"Separating Conjoined Twins" by Rabbi Chaim Steinmetz

"Conjoined Twins" (Audio) by Rabbi Ari Kahn

"Siamese Twins: Rav Feinstein’s Ruling and the Subsequent Controversy" By Mordechai Halperin M.D. http://www.daat.ac.il/daat/kitveyet/assia_english/halperin2-1.htm

"Split Decision: Separating Conjoined Twins in Halacha" Speakers include: R' Daniel Feldman, Mr. David Wassermen, and Dr. Gerard Weinberg.

"Separating Siamese Twins - An Ethical and Halachic Dilema" Speaker: Rabbi Dovid Gottlieb

"Yarchei Kallah 5761 (2001) - "THE SIAMESE TWINS DILEMMA ACCORDING TO HALACHA"" (Audio) By RABBI Z. WINNER

"Siamese Twins: So One May Stay Alive" By Nissan Dovid Dubov (Audio lecture)

Tefillin: בעא מיניה פלימו מרבי מי שיש לו שני ראשים באיזה מהן מניח תפילין א"ל או קום גלי או קבל עלך שמתא אדהכי אתא ההוא גברא א"ל איתיליד לי ינוקא דאית ליה תרי רישי כמה בעינן למיתב לכהן אתא ההוא סבא תנא ליה חייב ליתן לו י' סלעים איני והתני רמי בר חמא מתוך שנאמר פדה תפדה את בכור האדם שומע אני אפילו נטרף בתוך ל' ת"ל אך חלק שאני הכא דבגולגולת תלא רחמנא - .מנחות דף לז

For more, see here: https://www.facebook.com/groups/562089430558585/permalink/728803960553797/

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Welcome to MiYodea! Please include a summary of the links, in case they go dead. – Danny Schoemann Nov 1 '15 at 8:59
    
I Replaced the 1st link with content from the link in case of link rot (also, original link was to my post in a closed FB group). I have made sure that all links (except the last one which is to a thread in a closed FB group) have been archived either @ archive.org &/ archive.is – user262055 Nov 25 '15 at 8:18

The Shevus Yaakov chelek 1:4(mentioned in user262055's answer) deals with a pair of Siamese twins who are joined at the heads which makes them appear as though they have one wide head. He writes that they would both require to wear teffilin on both heads. They would both receive a portion of yerusha not just a joined one. They would also not be allowed to marry because of the issur of tashmish bfnei kol chai.

He brings a proof from Adam and Chava who were joined together as one body yet they were called two separate creations. Here is the actual text:

שו"ת שבות יעקב חלק א סימן ד

משא"כ במעשה שלפנינו שגופים מחולקים דודאי תרי גופי נינהו גם הפרצופים והראשים הם שנים רק שהם דבוקים בקדקדם ואין כל חדש תחת השמש שכן היה תחלת הבריאה כדאיתא בברכות ועירובין ובמדרשים דו פרצופין בראם שנאמר זכר ונקבה בראם ויקרא שמם אדם ואף על פי שהיה דבוקים כל הגוף אפ"ה מקרי שני בריאות כדכתיב זכר ונקיבה בראם ויקרא שמם אדם: אחור וקדם צרתני וא"כ כיון דשני בריאות ושני גופים נינהו א"כ פשיטא דלענין תפילין צריך כל אחד להניח תפילין בפני עצמו על ראשו דהיינו הקציצה על הראש במקום שער בין עינים והקשר מאחוריו נגד פרצוף פנים דידיה: ולענין ירושה פשיטא דיורש שני חלקים כיון דתרי גופ' נינהו ומ"מ אסורין לישא אשה דיש חשש איסור א"א כיון שצריכן לשכוב במטה אחת גם אסור לשמש בפני כל חי ומהאי טעמא אפי' אחד זכר ואחד נקיבה או אפי' שני נקיבות ובמקום שנושאין שתי נשים אסורים להנשא כיון דאסור לשמש בפני כל חי

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