Daf 53a middle.
The gemoro tries to prove shmuel (that you must do chalitza to the one wih the strongest zika) and make a teyuvta to reb yosef (who says that you should not spill mey boro.. I.e. you should not ruin the fresh yevomo from kehuna when you can ruin the spoiled yevomoh, who is already posul for kehuna)
How is this a teyuvta to reb yosef, maybe reb yosef only said this din when one of the yevomos was already a gerusha or a chalutza from a different marriage. Not when one of the 2 yevomos has a weaker zika, perhaps then he will be modeh to shmuel, and say that the chalitza pesulah consideration supercedes the yishpoch mey boro consideration?
Avrumi Hersh, London england
Shalom Avrumi,
I see two different possibilities to address your excellent question:
The first is Tosfos (DH Leima), who writes that the Gemara is not trying to prove both for Shmuel and against Rav Yosef. Rather, the Gemara means to prove either for Shmuel or against Rav Yosef. As you already articulated in other words, one could hold like Rav Yosef, but then in light of the Mishnah one must hold like Shmuel. (The other alternative, of course (although it is less relevant to your question), is that one can disagree with Shmuel, but then in light of the Mishnah one could not hold like Rav Yosef.) In other sources, by the way, this explanation is identified as being that of the Ri.
The second approach is Rabeinu Meir, found in the Tosfos Rabeinu Peretz (and Tosfos ha'Rosh, which I do not have on hand). He argues with Tosfos and maintains that the Gemara is indeed proving both for Shmuel and against Rav Yosef, because it can be shown (from Megilah 18b and Yeavmos 50a) that Rav Yosef disagrees with Shmuel.
I hope this helps!
Best wishes,
Yishai Rasowsky