“ויהי ביום הששי לקטו לחם משנה שני העמר לאחד ויבואו כל נשיאי העדה ויגידו למשה” (ט”ז,כ”ב)
The Gemara in Shabbos 117a learns from this passuk that on Shabbos there is an obligation לבצוע על שתי ככרות – to make המוציא on two loaves. The ט”ז (סימן תרע”ח סק”ב) is of the opinion that the mitzvah of לחם משנה is מדאורייתא. This is also the opinion of the חתם סופר. However most poskim rule that it is only a מצוה מדרבנן and the passuk is only an אסמכתא. The מגן אברהם (סימן רנ”ד סקכ”ג) even says אינה חובה כל כך – it is not such a real obligation. However everyone in Klal Yisroel has accepted it as a real obligation, like the opinion of most poskim.
The מחבר (סימן רע”ד סעיף א’) writes בוצע על שתי ככרות one makes המוציא on two loaves. The רמ”א adds שלמות – they must be whole. The גר”א holds they have to be at least the שיעור כביצה. Others hold it can be as small as a כזית. Many poskim hold it can even be less than a כזית, and one would have to eat from other bread to be מצטרף to the שיעור כזית.
The שערי תשובה brings different opinions, what is considered a חסר and what is considered a שלם. להלכה the opinion of the poskim is that if small pieces, crumbs, or part of the crust falls off it is still considered a שלם. לכתחילה as long as it is not missing 1/48 from the לחם it is also considered a שלם. The מחבר (סימן קס”ח סעיף ב’) writes that a person can join together two halves with a piece of wood or something else and it has a דין שלם. This is only if one cannot tell that it is two halves.
לחם which cannot be eaten because of an איסור cannot be used for לחם משנה. On ערב פסח שחל להיות בשבת, the חזון איש holds that one can use matzo for לחם משנה even though he is not allowed to eat it. The פרי מגדים says that פת עכום can also be used for לחם משנה. If one is allergic to one of the שלמים he can still use it for לחם משנה. If someone borrowed לחם and the owner is מקפיד that it be returned, it is possible it cannot be used for לחם משנה. As far as a frozen חלה there is a dispute amongst the poskim. The מנחת יצחק ור’ נסים קרליץ both hold that one can be lenient and use it for לחם משנה. Since it needs no other תיקון, and can be warmed up in a permissible way on Shabbos. Also בשעת הדחק it can be eaten frozen. However Reb Shlomo Zalman and the שבט הלוי are both stringent that one should not use frozen לחם for לחם משנה. Reb Shlomo Zalman says that if he has in mind to have a long meal and the לחם will thaw out, then maybe there is room to be lenient.
The משנה ברורה says if one doesn’t have a פת שלמה it is not מעכב. However the poskim say that one should still use two pieces, even if they are not שלמים. If there is only one שלם and no other bread, there is a מחלוקת what to do. Some poskim hold you should not cut the שלם into two pieces rather make the brocha on the שלם as is. Others are of the opinion that if the שלם was not yet brought to the table, then it should be cut in half in order to be מקיים the halacha of לחם משנה. Others hold that it should always be cut into two, because לחם משנה is a מצוה גמורה while a שלם is only a preference over a broken piece.
Women are also obligated in the mitzvah of לחם משנה, since they also had the נס of the מן. There are many poskim who are מלמד זכות why some women are not מקפיד on לחם משנה. However להלכה they are obligated just like the men.
The one who is בוצע should have כוונה to be מוציא all the other people with his brocha, and they should have כוונה לצאת. מדרך ענוה the בוצע should say ברשות רבותי even if he is the בעל הבית. The minhag of the חזון איש was not to say ברשות, rather before the brocha he would turn a little towards the one whom he was being מוציא.
There is a minhag that people will make their own brocha on the לחם after the בוצע has already made a brocha and cut the לחם משנה. As long as they also heard the brocha of the בוצע, it is still considered that they had לחם משנה. The earliest source for this minhag is the father of the קרבן נתנאל. This minhag is brought in the שו”ע הרב and he gives the reason that people are not careful to refrain from talking after the brocha of the בוצע. Others hold that even if one did not hear the brocha of the בוצע it is still considered that he had לחם משנה. However this is not the accepted practice and לכתחילה everyone should be יוצא with the brocha of the בוצע.
The אר”י ז”ל holds that one should have twelve לחמים by all three meals on Shabbos, corresponding with the twelve לחם הפנים of the בית המקדש. However the מנהג העולם is to have two חלות in the shape of a “ו” which equals the numerical value of twelve.
Theמחבר says that only one of the חלות needs to be cut. The דין of לחם משנה is only by the brocha. However the opinion of the רשב”א, is that both need to be cut for לחם משנה. This is also the opinion of the גר”א.
Prepared by R’ Avrohom Yehoshua Ziskind
 Sources:שו”ע, משנה ברורה,דרשו,פסקי תשובות