The obligation of "Lehem Mishneh" requires reciting the Beracha of "Ha’mosi" over two whole loaves of bread at all three Shabbat meals. It sometimes happens that a person does not have a second loaf, and so he wants to borrow one from his neighbor, promising to return it afterward. Does this suffice for fulfilling the requirement of "Lehem Mishneh," or does one have to be able to slice and eat both loaves of bread?
Rav Eliezer Waldenberg (Jerusalem, 1915-2006) addresses this question in his Sitz Eliezer (11:23), and writes that one may fulfill the "Lehem Mishneh" requirement with a borrowed loaf of bread. Firstly, he notes, nowhere is it written that one must own the bread over which he recites the Beracha on Shabbat. The obligation of "Lehem Mishneh" is derived from Beneh Yisrael’s experience in the wilderness, when a double portion of manna fell each Friday for Shabbat, and thus it should not make a difference whether or not one owns both loaves of bread. As for the fact that one is unable to eat the borrowed loaf, which the neighbor expects to receive back, Rav Waldenberg cites the ruling of the Peri Megadim (Rav Yosef Teomim, 1727-1792) that one who avoids eating Pat Akum (products baked by a non-Jewish baker) may nevertheless use a loaf baked by a non-Jew for "Lehem Mishneh." This ruling demonstrates that even if one is unable to eat the second loaf, it may nevertheless be used for "Lehem Mishneh." And although the Peri Megadim allowed using "Pat Akum" only when other bread is not available, Rav Waldenberg contends that this is because "Pat Akum" is considered forbidden, and thus viewed with a degree of disgust, by those who avoid eating it. A borrowed loaf, however, which is inherently permissible for consumption, may be used for "Lehem Mishneh" even "Le’chathila" (at the optimum level of Misva of observance). Rav Waldenberg draws further proof from the ruling of the Lebush Mordechai (Rav Mordechai Yaffe, 1530-1612) that when Ereb Pesah falls on Shabbat, one may use Masa Shemura for "Lehem Mishneh." Despite the fact that Masa Shemura may not be eaten on Ereb Pesach, it may be used as "Lehem Mishneh" – seemingly proving that bread which one may not eat is nevertheless suitable for "Lehem Mishneh."
It often happens that loaves or rolls which are baked together in the oven become attached to one another over the course of baking, and are then separated afterward. May such a loaf be used for "Lehem Mishneh," or is it considered incomplete, since it had been attached to another loaf and then separated?
The Shulhan Aruch rules in a different context, amidst his discussion of the Beracha of "Ha’mosi" (Orah Haim 168:4), that such a loaf is considered "complete," despite the fact that it had been attached to another loaf and then separated. The work Tosefet Shabbat applies this ruling to "Lehem Mishneh," as well, and maintains that the loaf in such a case may be used for "Lehem Mishneh." However, the Mahasit Ha’shekel (Rav Shemuel Kolin, 1724-1806) writes that this applies only if the loaves were separate when they were placed in the oven, and became attached subsequently. If they were attached from the outset, then the two loaves are viewed as a single loaf. Hacham Ovadia cites Poskim which disagree, and maintain that even if the loaves were put in the oven together with the intention that they would be attached, and they were subsequently separated, they may nevertheless be used for "Lehem Mishneh," as long as the separation is smooth and they appear as separate loaves. This is also the view of the Sitz Eliezer (11:24). In such a case, however, according to some authorities, it is preferable to cut the loaf from the other end, and not the end that had been attached to another loaf.
Machine-made Masot may be used for "Lehem Mishneh," despite the fact that they are initially made as a single, large entity and then separated into individual Masot. The Halachic authorities explain that since the Masot are perforated, and thus from the outset were intended to be separated, and the slicing is perfectly smooth such that the Masot appear whole and complete, they may be considered as complete "loaves." This is the position of both Hacham Ovadia Yosef and the Sitz Eliezer.
Summary: One may borrow a loaf from a friend or neighbor to use for "Lehem Mishneh." If two loaves were attached in the oven and subsequently separated, they may be used for "Lehem Mishneh." Likewise, machine-made Masot may be used for "Lehem Mishneh," despite the fact that they had initially been attached.
