why only 2 matzos used for seder by the Vilna Gaon

Question

Why did the Vilna Gaon only use 2 matzos for his seder? The Rif also did that-- why?

Answer 1

The machlokes is based on what is the definition of lechem mishna that is required at the seder after Yachatz breaks one of the matzos that is on the table. That is, the basic requirement (derived from Shabbos) is Lechem Mishna Shleimim - two whole "breads" (at least Lechatchila). As we see below, it appears that breaking the yachatz piece during the seder, would remove it from the status of lechem mishne.

Those that hold "three" at the seder say that the lechem mishna must be shleimim (whole). Once Yachatz has broken one of the matzos, we no longer have two matzos, we have 1.5 (or no longer two full pieces). We do not consider the broken piece as a full "lechem" for the purpose of lechem mishne (as explained by the Netziv).

The Mitzvah of Lechem Mishneh

Tosafot, Berachot 39b, s.v. V'Hilchita, write that on Shabbat, one should not cut the bread until after reciting the beracha of hamotzi in order to fulfill the mitzvah of lechem mishneh. It is implicit from the comments of Tosafot that the mitzvah of lechem mishneh requires that one should use two whole loaves. R. Ya'akov Reischer, Teshuvot Minchat Ya'akov, no. 12, suggests that even if there is a biblical requirement of lechem mishneh, there is no biblical requirement to use two whole loaves. It is sufficient to have only one whole loaf. The requirement that the second loaf be whole is only rabbinic in nature. R. Naftali Z.Y. Berlin, Meishiv Davar 1:21, suggests that there is no inherent requirement to use whole loaves. If the person who is partaking of the lechem mishneh breaks the bread immediately prior to reciting hamotzi, that bread is invalid for his lechem mishneh. However, if the bread is broken prior to the meal, that bread may be used for lechem mishneh. Tosafot, who caution cutting the bread prior to the recitation of hamotzi, are concerned about invalidating the bread for lechem mishneh by breaking it immediately prior to eating it.

The Vilna Gaon (and others) say that the critical point is to have lechem mishna and that the broken matza is still in that category. That is, we are eating Lechem Oni.

Three matzot or Two

I could not copy the text from this link, but it cites Pesachim 115b on the meaning of Lechem Oni, Rav Pappa (Berachos 39b) that we make the bracha on a whole piece and the broken piece because it is Lechem Oni, and Rav Abba that on Shabbos one must use two whole pieces.

Rashi (Pesachim 116a) says that Al Achilas Matza is made on the broken piece and one whole piece, but Hamotzi must be made on two whole pieces, just like Shabbos.

Rambam (Laws of Leavened and Unleavened Bread 8:6) says that both brachos must be made on the same two pieces (one whole and one broken)

Seder Matza - Lechem Mishna

It is a custom in most of Israel to have, on the Seder table, three matzos under a cover. Why three?

In the seuda (festive meal) we make "HaMotsi" over the matzos, and at any Shabbos or YomTov we need a second challa ("lechem mishne") when we make "HaMotsi" at the seuda. But why do we then need three matzos instead of two? Well, in the course of the seder, at "yachats" the middle matza is broken in two, and half removed (the "afikomen"). If we only had two matzos to begin with, we would end with one and a half, which is inadequate for the lechem mishne. Therefore we start with three. This is the opinion of Rashi, Tosfos, and the Rash, and it is the ruling in the Shulchan Aruch.

However not all commentators agree with this opinion. The Gemora, in Pesachim (115A), only says that one of the matzos must be broken (not necessarily the "middle one"), and the Rambam (Maimonides) and the Gr"a (Vilna Gaon) were of the opinion that there should only be two matzos. In fact, the Rambam thought it was incongruous that there should be two loaves of bread at Shabbos and all the other festive seudos, but a larger number (three) on Pesach, when the matza is supposed to be "lechem oni", the "bread of affliction"! Today the followers of the Gra in Jerusalem use only two matzos. So did Rabbi Moshe Feinstein (zatzal), and so does Rabbi Joseph Soloveitchik (zatzal).

Answer 2

Gemara Brachot 39b:

‏ אמר רב פפא הכל מודים בפסח שמניח פרוסה בתוך ‏^[1]‏ שלמה ובוצע מאי טעמא {דברים טז-ג} לחם עוני כתיב א''ר אבא ובשבת חייב אדם לבצוע על שתי ככרות ‏^[2]‏ מ''ט {שמות טז-כב} לחם משנה כתיב ... רבי זירא הוה בצע אכולא שירותא ‏
R`Papa said: All admit that on Passover one puts the broken cake under the whole one and breaks [them together].What is the reason? Scripture speaks of 'Bread of poverty' (Rashi writes that a poor man usualy eats from a slice) R`Abba said: On Sabbath one should break bread from two loaves... R`Zera used to break off for [on] the whole meal What is the reason? Scripture speaks of 'double bread'

[1] msh210 noted that literaly "betoch" is inside, and Soncino, following tosfot here translates "under". I found in Ritba (הלכות סדר ההגדה) in name of Yesh mirabotenu that this expression is more adapted to "placing on" (as in the plate) and doe's not give ruling.
[2]

1. Tosfot in Psachim (116a) comments the juxtaposition of the the statement (on slice and the statement on 2 breads) is aimed at teaching about a second whole Matsa (i.e. 2 whole and 1 slice).
2. Rashi on Gemara here wrote that Rabi Zera cuts only one of the two loaves of Lechem Mishne. Rashba is opposed to this comment and explain that R.Z. cut the all loaves on the table (He is followed by Rashal, Shla and GRA (O.C. 274 cited bellow).).

Tosfot (on the page) explain:

‏ ‏‏ הכל ‏‏ מודים בפסח שמניח הפרוסה בתוך השלמה ובוצע מאי טעמא לחם עוני כתיב. משום דכתיב לחם עוני מניח הפרוסה תחת השלמה ונראה כבוצע על הפרוסה ‏^[3]‏ ומ''מ אין לבצוע כי אם על השלמה ‏^[4]‏ ועל הפרוסה יברך על אכילת מצה ‏^[5]‏ וכורכים ואוכלים משתיהן יחד כדי שיהא נראה כבוצע על הפרוסה וכן המנהג ‏^[6]‏ אבל אין לעשות המוציא וגם על אכילת מצה מן הפרוסה דהוי כעושה מצות חבילות חבילות ‏^[7]‏ ונראה לי דהא הוי ברכה של נהנין ואינם נקראים חבילות חבילות דהא אקידוש מברכים קידוש וברכת היין ורבינו מנחם מווינ''א היה רגיל לעשות הכל על הפרוסה ואין השלמה באה כי אם בשביל לחם משנה ‏^[8]‏ והר''י היה רגיל לברך תחלה ברכת המוציא על שתיהן קודם שיבצע ואחר כך מברך על אכילת מצה ובוצע משתיהן אחר כך ואין זה חבילות מאחר שהיה עושה על שתיהן ‏^[9]‏ ופעמים מפיק הר''י נפשיה מפלוגתא ולא היה רוצה לשנות המנהג והיה מברך על השלמה תחלה המוציא והיה בוצע קצת ולא ‏^[10]‏היה מפרידה עד שיברך על הפרוסה על אכילת מצה ובוצע משתיהן יחד: ‏

[3] The slice is placed under the whole matsa and when he cuts with his hand the whol matsa it seems as if he broke from the slice. (Tosfot generally thinks that in Lechem Mishne we need to cut the both breads. So, Tosfot here, at this level thinks that in simple holliday we don't need Lechem Mishne. And the whole matsa is here to honor the bracha only.)
[4] Really he starts to cut into the whole.
[5] On the slice he blesses "Al Achilat Matsa".
[6] He eats the 2 together as if the bracha Hamotsi is on the slice.
[7] The reason we should not bless two Brachot on the slice, Hamotsi and Al achilat Matsa, is: Not to keep mitsvot "in the same package". So each Bracha is on a separate matsa. But Tosfot rejects the topic of "package of mitsvot" when the mitsvot are only blessings. In Kidush many blessings are on one glass of wine.
[8] Rabbi Menachem from Vienna (this opinion is reported in tos. Psachim 116a, and in Tos. Rabbi Yehuda Hachassid here). His opinion varies from the first of Tosfot in two points.

1. Firstly: in a simple Holliday, we need Lechem Mishne. Shabbat and Holliday are equals concerning the need of Lechem Mishne; because a double ration of manna was failing from heaven before hollidays.
2. Secondly: He makes both brachot on the slice. There is no need to honor the whole matsa with the Bircat Hamotsi in this particular case of Lechem Oni. The presence of the whole matsa is only to provide Lechem Mishne.
3. An additional point: R.M.V. thinks that despite the fact that one of the two is not entire, we have Lechem Mishne (contrary to the practice cited in the end of the Tosfot psachim (116a, also cited in Tur 475 quoted bellow) witch need 2 whole Matsot besides the slice)

[9] R.I. holds both when he blesses Hamotsi and Al Achilat Matsa, and then takes from both. The count of two Matsot corresponds to the count of Mitsvot. and no problem of "packets"
[10] At times, R.I. blessed Hamotsi and cracked the whole matsa a little (to honor the blessing with the whole), then blessed A.A.M and cracked both.

Tur O.C. 475, 7

‏ כתב רב אלפס שאין צריך אלא שתי מצות, אחת פרוסה לשנים ומברך על חציה המוציא ועל אכילת מצה וחציה השני לאפיקומן והשלמה לכריכה‏[11]‏ וכן כתבו קצת הגאונים ובחילוף מנהגים כתוב אנשי בבל כשחל פסח בשבת מניח פרוסה בין שתי השלימות וכשחל בחול מביא פרוסה ושלימה ומברך עליה ב' ברכות‏[12]‏ ואנשי ארץ ישראל בין בחול בין בשבת מניחין פרוסה על השלימה ומברך ‏(‏ על השלמה )‏ [על הפרוסה] שתי ברכות‏[13]‏ וכן חילק בה"ג שכתב והיכא דמקלע פסחא בליל שבתא בוצע על תרתין ופרוסה והתוס' כתבו שצריך לעשות ג' כדפרישית וכ"כ רב עמרם ולזה הסכים א"א הרא"ש ז"ל. ‏

[11] According to the Rif. 2 matsot: 1 matsa divided in 2: one half for Motsi and Matsa, the second half for Afikomen. The whole matsa for Korech.
[12] The citizens of Babylon think that the Mitsva of Lechem Mishne is on Shabbat only and not on Hollidays that fall on other days of the week. They think also that Lechem Mishne requires 2 whole matsot.
[13] The citizens of Erets Israel think that one doesn't necessarily need 2 whole Matsot for Lechem Mishne. So on Shabbat as on Chol, 2 matsot are sufficient.

The GRA added a reason, in Siman 473, 4:

If you have 2 whole matsot, adding a half matsa does not give the impression that you are poor. So, the lack of 2 whole matsot is imperative. He says also that Yom Tov and Shabbat are equals in their need of Lechem Mishne, because on Holidays the manna did not fall. And he decides that the opinion of Rif, Rambam, Magid Mishne and Rashba is the Halacha.

Summary, according to the Gaon

1. With more than one whole Matsa, we get Lechem Mishne (necessary for every Holliday).
2. Thanks to the lack of entirety of the second matsa, we don't lose the effect "Lechem Oni" of the slice.
3. He needs to break the two Matsot (according to his own opinion on S.A. O.C. 274, 1 regarding the general issue of Lechem Mishne)