Before us is Mishnah 6, which deals with a case where a person has two baskets of tevel in front of him, and he wishes the tithes of one basket to be located in the other. The Mishnah opens: "Hayu lefanav shtei kalkalot" - he had before him two baskets of tevel produce, from which Terumah and tithes had not yet been separated. The Mishnah then presents three formulations that a person might use to designate the place of the tithes, and each one carries a different ruling.
The first formulation - "ma'aserot zo bazo":
The person declares that the Terumah and tithes of basket A should be located in basket B. Thereby, "harishonah me'usheret" - basket A becomes tithed, since the tithe has been taken from it and it is no longer tevel, and the Terumah and tithes are now located in basket B.
The second formulation - "shel zo bazo v'shel zo bazo":
Here the person says that the tithes of basket A should be located in basket B, and the tithes of basket B should be located in basket A. In this case too, only "harishonah me'usheret": basket A becomes permitted produce, but basket B does not. The reason lies in the sequence of events - the statements were made one after the other, and consequently, at the moment the second statement was made, that the tithes of basket B should be located in basket A, basket A was already permitted produce and exempt from Terumah and tithes. And since it is exempt, it has no capacity to contain the Terumah and tithes being separated on behalf of basket B. The result is that basket B remains tevel, and its tithes have not been separated.
The third formulation - "ma'aserotehen ma'aserot kalkalah bachavertah":
Here the person does not speak in sequence, but rather designates everything in a single statement: the tithes of both baskets should be each basket in its counterpart, one basket in the other - and with this formulation it is effective, because the designation takes effect in both baskets simultaneously. The explanation is this: in each of the baskets we view it as though the quantity of produce fit to serve as tithe for the other basket had been separated, and the two separations take effect at precisely the same moment.
An additional explanation of the final case:
There is another way to explain the end of the Mishnah. According to the first explanation, each of the two baskets contains the Terumah and tithes of the other and serves as the separation on its behalf. According to the additional explanation, we view the two baskets as joined together as one, and the person may take the Terumah and tithes for both baskets from either one of them - both for basket A and for basket B - and the tithe takes effect in any event, from whichever one he chooses to separate.
In summary: In this Mishnah we learned three formulations regarding two baskets of tevel: when he says that the tithes of basket A should be located in basket B - the first is tithed; when he makes a double, consecutive statement of "the tithes of this one in that one and of that one in this one" - here too only the first is tithed, since once basket A became permitted produce it was exempted and can no longer contain the tithes of basket B; whereas with a single statement of "ma'aserotehen ma'aserot kalkalah bachavertah" - the designation takes effect in both baskets simultaneously, and both are tithed.