A kav of new [flour] and a kav of old [flour] that “bit” into one another, R' Yishmael says: He takes [challah] from the middle. But the Sages forbid [this]. [If] someone took challah from a kav [of dough], R' Akiva says: [It is] challah. But the Sages say: It is not challah.

Two [doughs] of a kav and a rice [dough] of a kav or a terumah [dough] of a kav in the middle do not combine. [If] something whose challah was taken [was] in the middle, they combine, since it was once obligated in challah.