Challah 4:3-4

Challah 4:3

If there are two pieces of regular grain dough each containing a kav of flour (about 1.25 liters each) and in between them there is a dough containing a kav of rice flour or a kav of terumah flour, the doughs of regular grain do not combine. If there is in between them another dough of regular grain from which challah has already been taken, the original two do combine because the intervening dough has already become obligated in challah.

Challah 4:4

If a dough containing a kav of “new” grain (chadash) connects to one containing a kav of “old” grain (yoshon), Rabbi Yishmael says that one should separate challah from the place where they meet; the Sages do not permit this (and require one to separate challah for them from other doughs). Rabbi Akiva says that if one did take challah from the intersection of the doughs, it is valid; the Sages say that it is not valid, even after the fact.