According to Rebbi Yosi, the reason why the upstairs resident does not pay is, as you said, because we are talking about a landlord and tenant. This is implicit in the Mishnah which mentions a landlord (Ba'al ha'Bayis) who does not want to fix the problem. The reason why the Mishnah mentions this case and not a case of two partners or other cases is because here the dispute is clear: Rebbi Yosi sees it as an issue of "straightening the floor," meaning adding a bit of plaster, which can be done by the tenant without requiring the landlord. The Rabanan view it as a more definitive problem in the roof which requires the intervention of the landlord, since it is his responsibility to maintain the strength of the roof for his tenant.

However, in the case of partners, or brothers who inherited, the rule is not clear, as there are four different opinions as to who is responsible. Some say the upper one is responsible, others say that everyone agrees in this case that the lower one must pay for the roof and the upper one for the plaster, some say the lower one is responsible for everything, and some say that this too is a dispute between Rebbi Yosi and the Rabanan.

According to the fourth opinion, your question does not begin, but even according to the second opinion it is obvious that the upper one must pay something as he is a partner and not a tenant.

According to the first and third opinions, there is no need to mention partners, as in such a case it is clear who is responsible -- either the upper one or the lower one (although it is unclear why these Rishonim are in dispute).

Yoel Domb