Q. For lehem mishneh are two shleimim required?
Does this apply to small chalos that are less than a kezais?
A. Shulchan Aruch (O.H. 274: 1) rules that one should break bread on the Shabbos seuda on two complete chalos. Mishna Berura (ibid. 1) quotes as the source the posuk mentioned by the monn (Shemos 16: 22); “On the sixth day that they gathered a double portion (lechem mishne) of bread.”
The complete chalos or matza used for lechem mishne, should be each at least egg full sized (kebeitza), but not less than the size of a kezais (olive or half an egg – Kaf Hachaim ibid. 8, Chikrei Leiv 32).
However, in need Piskei Teshuvos (ibid. 2) mentions that one may use those small breads or matza crackers for lechem mishne, and then complement the amount of a kezais also needed for birchat hamazon, fom other sliced bread.
Horav Shlomo Miller’s Shlit’a opinion is similar.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a