Responsa к Хала́ 2:11

Q. I was wondering at your method of taking Hallah from unleavened cakes; for, after they were baked you put all the cakes into a basket and took one cake as Hallah for the rest. It is true that in the Mishnah (Hallah 2,4.) R. Eliezer decides that cakes taken from different pieces of dough may be joined for the purpose of taking Hallah by putting them in one basket. But this law of R. Eliezer is only a restrictive measure in order to require the taking of Hallah from small pieces of dough (by putting them together in a basket), which are, by themselves, too small to necessitate the taking of Hallah from them. But R. Eliezer never meant his law to be a palliative measure. I, therefore, take a small piece from each quantity of dough that is separately mixed, and then join the small pieces into one piece as Hallah.
Signed: R. Isaac b. Simeon.
A. Throughout France Hallah is being taken from unleavened cakes by uniting them in a basket. This method is approved by the author of Sefer ha-Terumah, and the author of Eben ha-Ezer. But your method is not to be followed, for while the small pieces are being accumulated they become leaven.
SOURCES: Cr. 21; Pr. 322; L. 366; Rashba I, 834; Wertheimer 16; cf. Am. 57; Pr. 279.