Thurs Nedarim 46

The Gemara says that even R' Eliezer b. Yaakov permits if the courtyard is too small to split up between the partners. However, if they could split up, then he cannot enter the courtyard.

Ran explains: that we cannot say he bought the whole courtyard for when he uses it, since his partner can force him to split, we say that he may not bought this part that he's entering, since it might fall out in his partner's half after the split.

The Rashba explains: it would be Assur after the split too. Since the split is a sale (each one buying out the other one's partnership in their half.) We have a rule that even if the oath maker sells his house, the Assur party cannot go in unless that there after the fist sale, but only after the buyer sells it to a third person. However, the Rambam permits it after the split. The Ran explains: we must say that it's an unstipulated condition in the original partnership that each side cannot make an oath to forbid the partner after the split. Therefore, he never owned it enough to be able to forbid it.