Baba Bara 55b - Sfeik Sfeika by Tu'mah

Tosafos cites 3 different approaches to explain the machlokes between the Rabbonon and R' Elazar:

1. Rashbam - All agree that a safeik tu'mach in reshus hayachid is ta'mei. All agree that sfeik sfeika is tahor. The machlokes is whether the safeik of coming into the field that contains the tu'mah and the safeik of whether he came into contact with the tu'mah qualifies as one safeik, or as a sfeik sfeika. R' Eliezer considers it sfeik sfeika, whereas the Rabbonon consider it one safeik. The reason that the Rabbonon consider it only one safeik, the Rashbam writes - דסוף סוף מה לי שדה אחת מה לי ב' שדות, חדא ספקא היא וכו' וספקו טמא. The Rashbam seems to understand that the reason it is considered one safeik is like the "שם אונס חד הוא" logic that Tosafos uses in kesubos 9a - since it boils down to only one issue and that is whether there was contact with tu'mah we consider it only one safeik.

2. Rabbeinu Tam - The machlokes is whether the chiddush that safeik tu'mah in reshus hayachid is ta'mei, which we learn from sotah, applies even to a sfeik sfeika. Rabbonon hold that since it is a chiddush to be metamei in reshus hayachid, we are even me'tamei a sfeik sfeika. R' Elazar holds אין לך אלא חידושו and we are only metamei one safeik, but a sfeik sfeika would be tahor.

3. Rashi in Avodah Zarah 70a - The case is when there is only one safeik, there is no sfeik sfeika because the entire field is filled with tu'mah. If it were a sfeik sfeika then all would agree to be me'taher, but being that it is only one safeik there is a machlokes. R' Elazar holds that since the type of safeik we are speaking about is not a safeik of touching but a more removed idea of whether he even walked into the field, this is not the type of safeik that we are me'tamei in reshus hayachid, so we are me'taher. Rabbonon argue and hold that we are even me'tamei a ספק ביאה not just a ספק מגע.

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The Shach (y.d. 110 kuntres ha'sfeikos 14) discusses whether a sfeik sfeika must be מתהפך, meaning it may only qualify as a sfeik sfeika if it could be said in both directions starting from either safeik. But if it can only be said in one direction it wouldn't qualify as a sfeik sfeika. The Beis Meir (printed in margin of standard shulchan aruch) quotes our Tosafos to support a dissenting opinion that sfeik sfeika need not be מתהפך. From all the opinions in Tosafos it seems that we consider safeik bi'ah, safeik ma'gah, to be a sfeik sfeika. The sfeik sfeika will only work in one direction, but if we were to start with the safeik ma'gah, because once we say אם תמצא לומר נגע we no longer have any tzad to be metaher. Rashbam considers the point of argument to be whether this qualifies as a sfeik sfeika due to שם אונס חד הוא, but doesn't seem concerned with the fact that it is not מתהפך. Rashi has to make the entire field filled with tu'mah because all would agree that he is tahor relying on sfeik sfeika. Rabbeinu Tam also understands that it is a valid sfeik sfeika, the only issue is whether sfeik sfeika is tahor in a reshus hayachid, but in other contexts it would be a valid sfeik sfeika.