Q. If the complete and full (shleimos) challos used for lechem mishneh on Shabbos, are stale and hard, can I just eat a bit from one of them to comply with the hamotzi, and then eat the rest of the needed kezais from fresh but incomplete bread?
A. Horav Shlomo Miller’s Shlit’a opinion is that on the onset, one should eat a full kezais from one of the shleimos breads even if they are stale. However, in need the smallest shiur for a kezais would be accepted, while the rest is eaten from the fresh bread. When reciting hamotzi, the fresh bread should be held together with the two complete challos.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a