Not eating from the Lechem Mishnah, rather from the incomplete bread

Question

If someone only has one broken piece of bread, can he borrow from his neighbor two whole challas and make ha'motzei on everything together (the 2 challas and the piece of bread); however not eat from the challas and give them back to the neighbor?

I'm essentially asking if there is any "advantage" of making the bracha of ha'motzei with lechem mishneh even though one will not eat from the lechem mishneh at all.

Relevant information: We find in Hilchos Pesach (Shulchan Aruch Siman 482 Sif 1 in the Rema) that one who only has 3 whole matzas to use for both nights of Pesach he should break one of them and make all the brachos however only eat from the broken piece of matzah, leaving the whole ones for the 2nd night. Could this apply the whole year? Or perhaps Pesach is different since according to some Rishonim (at least one, the Rif?) a person makes ha'motzei on the broken piece of matzah.

Answer 1

If he is "borrowing" 2 Challos which he cannot eat he cannot make a Brocha on them and it is as if he is only holding the Broken bread as he can only make a Brocha on his own bread Mishna brura Orach Chaim 168,5: 21 comenting on this Shulchan Aruch:
פת עובד כוכבים נקיה ופת קיבר של ישראלמסלק פת נקי של עובד כוכבים מעל השלחן עד לאחר ברכת המוציא

ודע דכהיום שהמנהג שכל אחד מברך לעצמו ברכת המוציא לכאורה לא שייך כלל דין זה אלא כל אחד יברך על פתו שאוכל [מחה"ש]
One does not want to eat the white bread belonging to someone else eating together with him (its Pas Palter), when ones bread is black one would have to remove the white bread from the table, -Mishna Brura- but since everyone only eats their own bread (they don't share bread) and can anyway only make a Brocho on their own bread so one doesn't need to remove the white bread off the table

If he can eat one of the "borrowed" whole loaves he is obligated to, as he cannot eat a Prusa (broken) when a Shaleima (whole) is available, as already noted Shulchan Aruch Orach Chaim 168,1*

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*Pesach Seder night (mentioned by OP) is an exception as there is a Mitzva to specifically eat "lechem Oni" i.e broken peice of matza SA OC 473,4 so that's why even together with whole matzos we prioritize and eat the broken one, but this would not apply with regular eating bread on Shabbos on regular Shabbos.

Answer 2

I believe you are asking two sub-questions

1. Is there an advantage to saying hamotzi over two full loaves?
2. If that is the case, can one borrow bread from a neighbor to say hamotzi even if not eating the bread?

On the first question SA OC 274:1 writes that when saying hamotzi, one must be holding two whole loaves or rolls. The challot should ideally be full, without significant cuts, scrapes or burns. According to some poskim, even if one will not eat bread after Kiddush, but rather mezonos, he should hold two full pastries or baked foods together to fulfill the obligation of lechem mishne (Kitzur SA 77:17) (taken from R Daniel Braude's Learn Shabbos, p. 76).

On the second question, Shmirat Shabat KeHilchata 55:13 writes that if one is lacking a second loaf for lechem mishneh, one may borrow a loaf from a neighbor, say the blessing over bread, cut one's own loaf and then return the borrowed loaf to its owner.

However, the problem with your proposal is that, when one is in front of a whole loaf and a slice of bread, one eats from the loaf (Brachot 39b, SA OC 168:1, SA HaRav 168:1), therefore I don't think it is possible to borrow two loaves, eat from one's slice and return the loaves to one's neighbor.

Of course, consult your rabbi before implementing anything you learn here.