Nedarim, chapter five, mishnah two. This mishnah is built on the foundation of the previous one, in which the Tanna Kamma and Rabbi Eliezer ben Yaakov disagreed about whether we say bereirah: is it established retroactively that every time one of the partners uses the courtyard, he is using his own portion, so that the courtyard is considered his alone at that moment? The Tanna Kamma held that this is not so, since with every entry a person is entering a jointly owned courtyard; Rabbi Eliezer ben Yaakov, however, held: every time I enter, the courtyard is mine, and when you enter, it is yours.
Our mishnah deals with a case in which a third person is involved:
"Hayah echad min hashuk mudar b'echad meihem hana'ah - lo yikanes lechatzer" - what is the law regarding a person from the marketplace, that is, a third party, who is forbidden to derive benefit from one of the partners?
According to the Tanna Kamma: he may not enter the courtyard. Even if he is entering for the sake of the other partner, the one from whom he is not forbidden to derive benefit, it is still not permitted, because at the moment he enters he is also entering a courtyard belonging to the person from whom he may not derive benefit.
According to Rabbi Eliezer ben Yaakov: who disagrees here based on his approach in the previous mishnah, the one who is forbidden to derive benefit may say "l'toch shel chavercha ani nichnas" - I am entering the portion of your fellow, with whom I have no vow, and not your portion; for at the moment I enter, I am entering the domain of the person from whom I am not forbidden to benefit.
It is worth noting that there is an opinion that limits Rabbi Eliezer ben Yaakov's leniency: he permitted it only when the person enters for the sake of the partner with whom he has no vow, but entering the courtyard for no particular purpose was not permitted.
In summary: our mishnah applies the dispute about bereirah from the previous mishnah to a third person from the marketplace who is forbidden to derive benefit from one of the partners: according to the Tanna Kamma he may not enter the courtyard, while according to Rabbi Eliezer ben Yaakov he can say "l'toch shel chavercha ani nichnas" - and according to one opinion, specifically when his entry is for the sake of that partner.