How to hold the bread of Lechem Mishneh when saying Hamotzi on Shabbos and Yom Tov?
Two hands-Ten fingers: It is proper [anytime one says Hamotzi on bread] to place both hands [i.e. ten fingers] on the bread while saying Hamotzi, as they contain ten fingers corresponding to the ten Mitzvos that are associated with bread.
How to hold onto the loaves: The loaves are to be held one on top of the other. Those people who are accustomed to cut the lower loaf first on Shabbos night, are to place the lower loaf closer to them, than the upper loaf. Alternatively, [before] the blessing of Hamotzi, they are to move the upper loaf to the bottom, and then cut from it. [The above is in accordance to the revealed aspects of the Torah. However, according to the Kabala of the Arizal, the loaves are to be held side by side, back to back, having the flat sides of both loaves adjacent to each other, and the braided sides on the exterior, one side facing the left and another facing the right, thus appearing like a single loaf with two fronts. Practically, the Chabad custom follows this latter approach, and so was the custom of the Rebbe. According to Chabad custom, on Shabbos night, the loaves are to be equally side by side, with no one loaf protruding higher than the other. However, by daytime, the right loaf is to protrude slightly higher than the left. ]
Which bread to cut: There is no obligation to cut both breads. [According to the Halachic approach of holding the bread one on top of the other] one is to [always] cut the upper loaf first. [According to all], this is followed by the Shabbos day meal and the Yom Tov night [and day meal]. However, regarding the Shabbos night meal, some people are accustomed for reasons known to them, to first cut the lower loaf. Those who follow this custom, are to either position the lower loaf closer to them or move the upper loaf to the bottom after the blessing is said, as mentioned above. [The above follows the Halachic positioning of the loaves, as explained above. However, according to the Kabalistic positioning of the loaves, to hold them back to back, one is to always cut the right loaf first.]
Both loaves must be held in one’s hands at the time of Hamotzi. One is to hold on to them with two hands, and all ten fingers. There exist two customs regarding the positioning of the bread: 1) One on top of the other. 2) Side by side, and back to back. According to the first custom, there are various approaches as to which bread is to be cut first, and which bread is to be closer to the person. According to the second custom, the right loaf is to be cut first.
Chabad custom: The Chabad custom is to always place the loaves back to back, and side by side, as explained above, although on Friday night the two loaves are equal in height, while Shabbos day, the right loaf is slightly higher than the left. One always cuts the right loaf after Hamotzi.
Should a lefty cut the left Challah first?
According to the custom of Arizal/Chabad to hold the Challahs side by side, are they to be held standing up [vertically] or horizontally?
 Admur 274:2
 Admur 167:7; Michaber 167:4; Rokeiach 329; Shivlei Haleket 141
 Some Poskim rule one is to hold it in a way that one places eight fingers on top of the bread, and the two thumbs under the bread. [Maharil brought in M”A 167/2]
 The ten Mitzvos: These ten Mitzvos are: 1) Not to plow with a ox and donkey together; 2) Not to plant Kiolayim; 3) Leket; 4) Shicha 5) Peiah; 6) Bikurim; 7) Terumos; 8) Maaser Rishon; 9) Maaser Sheiyni; 10) Challah. [Admur ibid; Tur in name of Rokeiach]
The ten words in Birchas Hamotzi: It is for the above reason that there are ten words in Birchas Hamotzi, and ten words in the verse Matzmiach Chatzir Labeheima, and ten words in the verse Einei Chol Eilecha Yisabeiru, and ten words in the verse Eretz Chita. [Admur ibid; Michaber ibid] And ten words in the verse of Vayiten Lecha. [Michaber ibid; omitted in Admur ibid]
Kabbalistic reason: The ten fingers represent the ten Sefiros that draw Elokus into Malchus. [Kaf Hachaim 167/32 and 34]
 Admur ibid; Michaber 274:1; Rav Kahana Shabbos 117b
 Admur ibid in parentheses “It is not necessary to hold onto both loaves when breaking a piece off from the bread, and he may thus let go of it, as it suffices for him to have been holding it during the blessing of Hamotzi.”; Admur 475:4-5 regarding Motzi Matzah
 The reason: As the verse states “Laktu Lechem Mishneh.” This comes to teach us that [it does not suffice to simply have the two loaves on the table but] that one must hold on to the loaves [when saying Hamotzi].
 So is evident from all Poskim [Admur ibid; Michaber ibid; Rama, Taz, and M”A 274:1] who discuss cutting the upper or lower loaf; Torah Or Beshalach p. 66; Likkutei Torah Eikev p. 17 that so is according to Nigleh
 Custom in Admur ibid; Rama ibid; Michaber ibid regarding all meals; See Levush 274:1; Olas Shabbos 274:1
 Admur ibid; Taz 274:1
 Admur ibid; M”A 274:1
The reason: As one may not pass over a Mitzvah [which is the upper loaf] and thus in order to avoid this [prohibition], they place the lower loaf closer to oneself, and it is therefore found that they are not passing over any Mitzvah at all. [Admur ibid; Taz ibid]
 Torah Or Beshalach p. 66; Likkutei Torah Eikev p. 17 that so is according to Nigleh; However, see Rama ibid and other Poskim ibid who explain even according to Kaballa, that one loaf is on top of the other. Perhaps then the intent of Admur ibid is to the Kaballa of the Arizal.
 Arizal in Peri Eitz Chaim Shaar Shabbos 17 and Shaar Hakavanos, brought in Kaf Hachaim 262:2; Likkutei Torah Eikev p. 17; Siddur Im Dach p. 111; Mishnas Chassidim Leil Shabbos 4:2; Or Letziyon 2:21-1; Piskeiy Teshuvos 274:4 that so should be done
The reason: When the two breads join each other, back to back, it appears like one loaf with two fronts, and is similar to the Lechem Hapanim. [Arizal ibid]
 Igros Kodesh 18:361, printed in Shulchan Menachem 2:84
 Shulchan Menachem 2:84; See Hamelch Bemisibo 2:144
 Igros Kodesh 18:361, printed in Shulchan Menachem 2:84; See Piskeiy Teshuvos 274:4 footnote 34 that the right Challah is to always be closer
 Admur ibid; Michaber 274:1; Rav Kahana Shabbos 117b
The reason: As the verse only requires Lechem Mishneh by “Lekita” [the gathering or holding, and not by the cutting]. [Admur ibid; Rav Kahana ibid]
 Admur ibid; Bach 274 in name of Gedolim and Zekeinim, brought in Taz 274:1 and M”A 274:2; Kol Bo 24; Hagahos Maimanis Hilchos Chametz 8; Kaf Hachaim 274:2 that according to the Kaballa of the Arizal, one must always cut the upper loaf by all meals, and so is the final ruling, as who can argue on the Arizal who received from Eliyahu Hanavi; Nimukei Orach Chaim 274:3
The reason: As one may not pass over a Mitzvah [and since the upper loaf is closer to oneself, it is therefore not to be passed over]. [Admur ibid; Bach ibid; Yuma 33a]
Other opinions: Some Poskim rule one is to [always] cut the lower loaf first. [Michaber 274:1; Beis Yosef 274 in name of Gedolim and that so is proper according to Kabbala; Kol Bo in name of Yeish Nohagin; See Machztis Hashekel 274:1] Other Poskim were accustomed to cut both loaves. [Rashal, brought in Shlah, brought in M”A 274:1] The Bach ibid argues on the ruling of Michaber ibid, as it contradicts the Talmudic ruling of not surpassing a Mitzvah, and whenever there is a contradiction between Halacha and Kabbala, we rule like Kabala.
 Admur ibid; Rama 274:1; See Mishnas Yosef 5:69 that this applies even when Yom Tov falls on Shabbos
 Custom in Admur ibid; Rama ibid; Michaber ibid regarding all meals
The reason: This is due to reasons mentioned in Kaballah. [Rama ibid; See Beis Yosef ibid; Levush 274:1; Olas Shabbos 274:1
Machatzis Hashekel ibid]
 Arizal in Shaar Hakavanos, brought in Kaf Hachaim 262:2; Or Letziyon 2:21-1; Piskeiy Teshuvos 274:4
 Orchos Rabbeinu 1:113; Piskeiy Teshuvos 274:4 footnote 34
 Otzer Hashabbos p. 61 in name of Yitav Leiv; Piskeiy Teshuvos 274:4
 Or Letziyon 2:21-1