Challah, Chapter 4, Mishnah 1. This mishnah deals with the question of what creates a measure: when do two separate batches of dough combine with one another to complete the amount that obligates in challah, and when do they not combine.
Two batches of dough belonging to two women:
"Shtei nashim she'asu shnei kabin" - two women who each made a dough of one kav, so that together there are two kabin here. The amount that obligates in challah is five quarters of a kav, while a single kav is only four quarters; it turns out that each woman made a dough that does not contain the amount that obligates in challah.
"Venaga zeh bazeh, afilu hem mimin echad - peturim" - even though the doughs touched one another, and even if they are from two flours of the same type, they do not combine into the two kabin that would obligate in challah, because they belong to two different owners. What is more: even if they had kneaded them together it would not have helped, since the doughs are destined to be separated, each woman owning her own kav, and therefore they are exempt from challah.
Two batches of dough belonging to one woman:
When both batches of dough belong to a single woman, the law depends on the type of flour:
"Min bemino - chayav" - when the two doughs are of the same type (and in the next mishnah we will clarify the definition of 'the same type'), they combine and become obligated in challah, and it is enough that they adhere to one another, meaning that they touch and stick together to the point that if one were to pull them apart they would not separate cleanly, but a bit of dough from each would remain in the other.
"Veshelo bemino - patur" - when it is not the same type of flour, the doughs do not combine through mere touching and adhering, and they are exempt.
However, as we already mentioned in the first mishnah of the tractate, if both kabin belong to one woman and were fully kneaded together, then even if they are from two different types, they combine to make up the measure and are obligated in challah.
In summary: two batches of dough belonging to two women do not combine, even when they are of the same type and even when they were kneaded together, because they are destined to be separated. Two batches of dough belonging to one woman: of the same type they combine through adhering alone, and of different types they do not combine through adhering, but only through full kneading.
In the next mishnah we will deal with the definition of 'the same type': which kinds are considered one type and which are considered two types.