At the onset of the Bnei Yisrael’s journey through the midbar, we read in this week’s parshah that the Bnei Yisrael’s complaint was for food to eat. Hashem responded that He would send “lechem min hashamayim” (also known as mun), and that the Bnei Yisrael would collect each day’s portion according to the number of members of his household. One was not allowed to leave over any mun for the next day, and if he would it would spoil. On the sixth day a double portion would fall and the second portion was to be saved for Shabbos, as the mun would not fall on Shabbos. This double portion would not spoil, even though it was left over to the next day.
The Gemara in Shabbos 117b derives from the pasuk that mentions the double portion of mun that fell for Shabbos that one is obligated to “break” bread on two loaves of bread on Shabbos. Rashi explains that the obligation is only to recite the berachah of “hamotzi lechem” on two loaves; one need not break both loaves. The Gemara relates a story in which Rav Kehana held two loaves while reciting the berachah and only broke one of the loaves. The Gemara then relates another incident in which Rav Zera broke enough bread for the entire meal. The interpretation of the second incident is the subject of a dispute between Rashi and the Rashba. Rashi explains that it is an unrelated episode that teaches us that Rav Zera would break a bigger piece of bread than usual in honor of Shabbos. The Rashba explains that the Gemara is offering a disputing opinion to the previous one of Rav Kehana. Rav Kehana would only recite the berachah on the two loaves, and Rav Zera would even break both of them.
In other words, according to Rashi, one must only recite the berachah of hamotzi on two loaves and one need not break both of them; in his view this is not a matter of dispute. According to the Rashba, this is the opinion of Rav Kehana. Rav Zera disagrees and says that one must break both loaves as well. The Rashba adds that Rav Hai Gaon ruled that since the Gemara did not rule in this matter between Rav Kehana and Rav Zera, one is free to do as he wishes.
The Shulchan Aruch (Orach Chaim 274:2) rules in accordance with Rashi’s view that one must only recite the berachah on the two loaves, and that one need not break both of them. Also, that it is a mitzvah on Shabbos to break a big piece that would suffice for the entire meal. The Vilna Gaon, in the abovementioned siman, agrees with the Rashba that one should break both loaves of bread. Similarly, the Magen Avraham cites the fact that the Maharshal was accustomed to breaking both loaves of bread as well.
I would like to discuss the basis for this machlokes, and explain on a deeper level where they disagree. According to Rashi everyone agrees that one must only recite the berachah on the two loaves. On this opinion, the halacha that one must have lechem mishneh is similar to that of the daytime Kiddush. There is an obligation that the seudah begins with a berachah on lechem mishneh. Therefore, the following chiddush halacha should result: Just the same as Kiddush Rabbah (by day), only one person is required to recite the berachah, and everyone can fulfill their obligation without even drinking from the wine. So too only one person must recite the berachah on the lechem mishneh, and everyone will have fulfilled the obligation of lechem mishneh. This applies even if they do not eat from the lechem mishneh; rather they can eat from their own bread and make their own birchas hamotzi on it.
The Eishel Avraham (Reb Avraham from Butchatch found this in the back of the Shulchan Aruch, Orach Chaim 274) adds that one can even be yotzi his obligation of lechem mishneh if he hears the berachah before he washed his hands, and that the washing and reciting of its berachah will not be an interruption. Even though he will recite his own berachah of hamotzi, it is sufficient that he heard the berachah that was recited on the two loaves.
The Rashba explained that Rav Zera disagrees with Rav Kehana, saying that one must break both loaves. In fact he makes no mention of the berachah at all. In his view one must break both loaves and need not recite the berachah over both of them. According to this opinion the obligation to break both loaves of bread is a part of the seudah, and the seudah should be eaten with both loaves broken. One need not eat from both loaves, but we see that it is a part of the seudah. According to this opinion one would have to eat from one of the two broken loaves. It would not suffice to merely listen to the recitation of the berachah over them.