Demai, Chapter Six, Mishnah Seven
This mishnah deals with a case where two people gathered their grapes into one winepress. One of them tithes and the other does not. The question is: how much of the wine does the one who tithes have to tithe, all of the wine, or only the share that he ends up taking?
Two gathered [the fruit of] their vineyards into one winepress, one of whom tithes and the other does not tithethe one who tithes may tithe his own share and his share wherever it may be.
The one who tithes must separate tithes both for the share of the wine that he actually takes for himself, and for his actual share, wherever that may end up. In other words, if the wine that comes from his grapes ends up in the other persons hands, and the other person doesnt tithe, it will turn out that it is as if he sold the other person untithed produce. Above in mishnah 2:2 we learned that one must tithe the produce that one sells to someone who is known not to tithe.