The Misva of "Lehem Mishneh" requires reciting the Beracha of "Ha’mosi" over two complete loaves of bread at each of the three Shabbat meals. The Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) cites the opinion of the Bet Menuhah that although one is required to eat a Ke’zayit of bread at each Shabbat meal, the loaves do not have to be the size of a Ke’zayit. One needs to recite the Beracha over two whole loaves, regardless of their size. On the other hand, Rav Haim Palachi (Turkey, 1788-1869) maintained that each loaf must be at least the size of a Ke’zayit. As for the final Halacha, Hacham Ovadia Yosef ruled that since the requirement of "Lehem Mishneh" was instituted by the Sages, and does not constitute a Biblical obligation, one may rely on the lenient position and use loaves that are smaller than a Ke’zayit. Preferably, however, one should use loaves that are at least a Ke’zayit.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) ruled that one must hold both loaves not only while reciting the Beracha of "Ha’mosi," but also while cutting the loaf he is eating. Specifically, the Ben Ish Hai writes, one should hold the second loaf while cutting the slices for himself and his wife. This was the view of the Arizal (Rav Yishak Luria of Safed, 1534-1572). Others, however, including Rav Sheneur Zalman of Liadi and the Elya Rabba, ruled that one may let go of the second load after reciting the Beracha, before beginning to cut slices, and this is the view accepted by Hacham Ovadia Yosef (Yabia Omer, 8:32, in a footnote). Hacham Ovadia adds, however, that those who wish to follow the stringent opinion, and continue holding the second loaf while slicing the first, are deserving of blessing.
Hacham Ovadia also ruled that if a person does not have two complete loaves, and has only one loaf and another half a loaf, he should recite the Beracha over both. Based upon a Talmudic passage in Masechet Berachot, Hacham Ovadia notes that Halacha sometimes treats 1.5 entities as two entities, and thus with regard to "Lehem Mishneh," too, if one does not have a second complete loaf, he should use half a loaf. Furthermore, Hacham Ovadia maintained that if one does not have a second loaf but has a complete "Mezonot" food, such as a whole cake, he should use the cake for his second loaf to fulfill the Misva of "Lehem Mishneh." A person who eats a very large quantity of "Mezonot" food recites "Ha’mosi," and thus, in a certain sense, "Mezonot" food has the status of bread. As such, if one does not have a second loaf of bread, he should use a whole "Mezonot" food for "Lehem Mishneh."
Hacham Ovadia also cited the novel view of Rav Tzvi Pesach Frank (Jerusalem, 1873-1960) that if a person toasts a slice of bread, it is considered as having been baked anew. Therefore, it is no longer regarded as a piece removed from a loaf, but rather as a whole "loaf" unto itself, and may be used for "Lehem Mishneh." Based on Hacham Ovadia’s opinion elsewhere that it is permissible to toast bread on Shabbat – such as by placing it on a "Blech" – Rav Tzvi Pesach’s ruling offers a useful solution for cases where one does not have two whole loaves for "Lehem Mishneh." He can simply take a slice of bread, toast it, and then use it as his second loaf. Rav Shlomo Zalman Auerbach (Israel, 1910-1995) disagreed with Rav Tzvi Pesach’s ruling, and maintained that a toasted slice of bread does not have the status of a whole loaf. In any event, Hacham Ovadia ruled that one who finds himself without a second loaf may rely on this solution.
The Kisur Shulhan Aruch (Rav Shlomo Gantzfried, 1804-1886) ruled that if a person is having Kiddush with some "Mezonot" food before his Shabbat meal, as people often do in the synagogue on Shabbat morning, he should recite "Mezonot" over two pieces of food. Since one is eating "Mezonot" food as his "meal," he bears an obligation of "Lehem Mishneh" and must therefore recite the Beracha over two "loaves." Hacham Ovadia, however, disagrees, and maintains that since one will be reciting "Ha’mosi" at home over two loaves of bread, he is not under any obligation of "Lehem Mishneh" at the Kiddush. Nevertheless, Hacham Ovadia adds (in a footnote in Hazon Ovadia) that one who observes this stringency is worthy of blessing.
Summary: One should preferably ensure that both loaves over which he recites the Beracha of "Ha’mosi" on Shabbat are at least the size of a "Ke’zayit." One may put down the second loaf immediately after reciting the Beracha, though according to some authorities, one should continue holding the second loaf while slicing pieces for himself and his wife. One who has only a loaf and another half a loaf should use the half-loaf for his "Lehem Mishneh." One who does not have a second loaf of bread but has a "loaf" of "Mezonot" food, such as cake, should use the "Mezonot" food for "Lehem Mishneh." Additionally, one who does not have a second loaf can toast a slice of bread by placing it on a "Blech," and then use that slice as his second "loaf." According to one opinion, if a person is having Kiddush with some "Mezonot" food before his Shabbat meal, as people often do in the synagogue on Shabbat morning, he should recite "Mezonot" over two pieces of food.
