R' Eliezer says: [Challah] may be taken from a tahor [dough] on behalf of a tamei [dough]. How? [If one has] a tahor dough and a tamei dough, he takes an amount sufficient for challah from the dough whose challah has not been taken, and places less than a beitzah [of dough] in the middle, so that he will be taking [challah] from something in proximity [to the doughs]. But the Sages forbid [it].