Take for example the following case. Someone only has one piece of bread. However his neighbor has 2 whole challas (and only 2.) May the first with the piece of bread take from his neighbor the 2 whole challas and make on everything together (the 2 challas and the piece of bread) ha'motzei. 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 someone will not eat from the lechem mishneh at all?
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.