Demai, Chapter 6, Mishnah 7. This Mishnah deals with two men who are partners in a vineyard, where one of them is trustworthy regarding tithes and the other is not.
The case before us:
Two partners harvested their vineyards into a single winepress, into a single olive press:
One of them tithes - he is trustworthy regarding tithes and separates them properly.
The other does not tithe - he is not trustworthy regarding tithes, and anything of his is considered demai.
After the harvest they will divide the wine between them, each one taking his portion. About this the Mishnah rules: "HaMe'aser me'aser es shelo" - the trustworthy partner separates tithes from his own portion, "v'chelko b'chol makom shehu" - and also from his portion that is in his fellow's hands, wherever it may be.
"Ein berirah":
The basis of the ruling in this scenario is the principle of ein berirah, that there is no retroactive clarification. According to this principle, we do not say that whatever a person receives at the end of the division was designated as his from the outset. Rather, the partnership applies to every single part of the wine - even every molecule in his portion is half his and half his fellow's.
The practical implication:
Since we do not say berirah, the trustworthy partner must treat half of his portion as something from which tithes were never separated, since fifty percent of it truly belongs to his fellow who does not tithe. His obligation to separate therefore covers:
His entire own portion - fifty percent of the wine, for which he is responsible to separate tithes.
And his fellow's portion - half of what he takes for himself is considered to have belonged to his fellow from the outset, and therefore it too requires separation.
The result is that the scope of his obligation reaches seventy five percent.
In summary: In this Mishnah we learned about two partners in a vineyard who harvested into a single winepress, where one tithes and the other is not trustworthy regarding tithes. Because we do not say berirah, the division does not reveal that the portion he took was his from the outset; rather, every single drop is shared by both of them. Therefore "HaMe'aser me'aser es shelo" - both for his full portion and for his portion wherever it may be, and the measure of what he must separate comes to seventy five percent.