Eruvin, Chapter 6, Mishnah 5. This mishnah deals with shitufei mevo'os, the joining of a shared alleyway. First, an introduction: unlike eruvei chatzeiros, shitufei mevo'os need not be made specifically with bread. Eruvei chatzeiros can be made only with bread, a loaf, whereas shitufei mevo'os may be made with other kinds of food and drink as well.
The discussion in the mishnah:
The mishnah discusses a case in which, instead of the neighbors joining together and each contributing food for the shitufei mevo'os, they were already partners with one another in a food or a drink. Can they rely on that existing partnership?
Partnership in wine kept in a single vessel - a homeowner was a partner in wine together with his neighbors, the very neighbors with whom he must make shitufei mevo'os, and all the wine is kept together in one vessel. In this case the mishnah rules: "einam tzerichim eruv" - they do not need an eruv.
Partnership in wine with one neighbor and in oil with another - these are two different kinds of food, and they are necessarily kept in two separate vessels. And even if both partnerships were in wine, but the wine was in two vessels, that too is a problem. Therefore "tzerichim le'arev" - they must join together: they must form a partnership in some food or drink specifically for the purpose of shitufei mevo'os.
A point of precision: the "eruv" spoken of here is not eruvei chatzeiros, since eruvei chatzeiros require bread specifically. The term is borrowed, and its meaning here is shitufei mevo'os, the joining together of the various courtyards that open into the alleyway.
The opinion of Rabbi Shimon:
Rabbi Shimon disagrees and holds that whether both partnerships are in wine, or one is in wine and one in oil, they may rely on the existing partnership and use it as their shitufei mevo'os.
In summary: in this mishnah we learned that shitufei mevo'os may be made even with foods and drinks other than bread, and therefore an existing partnership in wine kept in a single vessel exempts the neighbors from making a shituf at the outset; but when the partnership is in two different kinds or in two vessels, they must join together, while according to Rabbi Shimon the existing partnership suffices in any case.