Demai, Chapter 6, Mishnah 9. This mishnah deals with a chaver - one who is trusted in matters of tithes and purity - and an am ha'aretz who inherited from their father together, and the question of how the produce they inherited may be divided.
This subject was actually already discussed in the previous mishnah, which included one line referring to this case: "or if they inherited." The Tosafos Yom Tov cites the Rosh, who explains that the mishnah was taught here only for the sake of the contrast with the next mishnah (mishnah 10), which deals with a convert and his gentile brother regarding their inheritance. It emerges that our mishnah comes only for the sake of the contrast, while its own subjects were already explained in the previous mishnah.
The permitted division - dividing within the same type:
"Chaver v'am ha'aretz she'yarshu es avihen am ha'aretz" - two brothers, one a chaver and the other an am ha'aretz, who inherited from their father who was an am ha'aretz. "Yachol hu lomar lo: tol atah chittim she'b'makom peloni, va'ani chittim she'b'makom peloni" - the chaver may say to his brother the am ha'aretz: you take the wheat in this area of our father's estate, and I will take the wheat in the other area.
The reason, as was explained in the previous mishnah: since the produce is attached to the ground, we say that the matter is clear and defined on its own - the portion the chaver received is the portion that was designated for him from the outset, while the portion his brother took was never his at all. Therefore he bears no responsibility to separate tithes from what his brother received.
And so too with wine: "Atah yayin she'b'makom peloni, va'ani yayin she'b'makom peloni" - you take the wine of this place, and I will take the wine produced from the grapes of the other area.
The forbidden division:
"Aval lo yomar lo: tol atah chittim va'ani se'orim" - each of the brothers has a share in each of the types. For that reason, when the chaver hands his brother the barley, he is giving him fifty percent of what is supposed to be his own, and therefore he bears the responsibility to separate tithes before transferring it to his hands.
"Tol atah lach va'ani yavesh" - this too the chaver may not say. "Moist" means produce that has become susceptible to tumah, and it may not be handed over to an am ha'aretz, who will thereby render it impure; while the chaver would take the dry produce for himself, which does not become impure. On this point the discussion is not about tithes, but about matters of tumah and purity.
In summary: In this mishnah we learned that a chaver and an am ha'aretz who inherited from their father who was an am ha'aretz may divide between themselves within the same type - wheat against wheat and wine against wine - since the portion each of them took is defined as his own from the outset, and the chaver is not obligated in tithes for his brother's portion. But he may not divide one type against another, since in doing so he transfers to his brother half of what belongs to him and must separate tithes beforehand, and he also may not hand him the moist produce that has become susceptible to tumah.
In the next mishnah we will deal with a convert and his gentile brother regarding their inheritance, which is the contrast for whose sake this mishnah was taught.