Reference for Demai 7:6

הָיוּ לְפָנָיו שְׁתֵּי כַלְכָּלוֹת שֶׁל טֶבֶל, וְאָמַר, מַעַשְׂרוֹת זוֹ בָזוֹ, הָרִאשׁוֹנָה מְעֻשָּׂרֶת. שֶׁל זוֹ בָזוֹ וְשֶׁל זוֹ בָזוֹ, הָרִאשׁוֹנָה מְעֻשֶּׂרֶת. מַעַשְׂרוֹתֵיהֶן מַעַשְׂרוֹת כַּלְכָּלָה בַחֲבֶרְתָּהּ, קָרָא שֵׁם:

If a man has two baskets of untithed produce before him and says, "The tithes of this one are in the other one," the first is tithed. [If he says,] "The [tithes of] this one are in the other one and of the other one are in this one," the first is tithed. [If he says,] "The tithes of each of them are the tithes of the other basket," it is a valid designation.

Jastrow