I am sure I am missing something obvious here, but I'm going to post this question anyway because I can't figure it out. The gemara always to the best of my recollection uses sfeik sfeika as a kula, but sometimes it can lead to a chumra as well. Whether or not you can apply a sfeik sfeika l’hachmir seems to depend on the machlokes Rambam and Rashba with respect to sfeika d’oraysa l'chumra. According to the Rambam, sfeika d’oraysa l’chumra is only a din derabbaban. When you add an additional safeik into the mix (a safeik whether you have a safeik) you really have a sfeika derabbanan l’kula. By this logic, S”S can only result in a kula, but never a chumra. However, according to the Rashba, sfeika d’oraysa l’chumra is a din d’oraysa. The reason sfeik sfeika works is because it tilts the odds -- instead of a 50-50 safeik, a sfeik sfeika creates a 75-25 safeik, which is a rov. One we are using the principle of rov, sfeik sfeika would seem to work l'chumra as well (Pnei Yehoshua Kesubos 9).
Some of the Achronim bring proof that we pasken sfeik sfeika l’hachmir even by hilchos brachos from the Beis Yosef in hil sefiras ha’omer (489) that if you have a safeik whether or not you counted on one night, you can count the remaining nights with a bracha because of a sfeik sfeika: safeik whether or not you counted at all, and even if you did not count, safeik whether or not each night is its own mitzvah and you can make the bracha anyway. The sfeik sfeika obligates you to keep counting and to say a bracha.
R’ Ovadya in the Yechaveh Da’as (5:21) brings other Achronim (the Nishmas Adam and others) who reject this proof. They point out that the makor of this Beis Yosef is the Terumas haDeshen, and the Terumas haDeshen writes mefurash that he is assuming that sefiras ha’omer is a din d’oraysa. You can’t bring a ra’aya from there to sfeik sfeika l'hachmir by a din derabbanan.
I don’t understand the logic. Even if sefiras ha’omer is a din d’oraysa, the bracha on sefira is only a takanah derabbanan! The proof from the B”Y is not that you can keep counting, it’s that you can keep counting with a bracha. Again, I'm sure I must be missing something, but I don't see what.
A few other parenthetical points: 1) we once discussed the Aruch haShulchan O.C. 175's proof that the B"Y holds sfeik sfeika l'chumra by brachos. It's a peleh to me that R' Ovdaya does not quote this mareh makom (it could be he discusses it elsewhere -- he touches on this topic of s"s is other places); 2) we discussed s"s l'hachmir in a different sefirah context here; 3) the proof from hil techumin quoted in that post can be countered with the argument that it's really only one safeik: is the boat in a location that is subject to the issur of techumin or not; 4) abother makor for s"s by brachos -- M"B in O.C. 215 with respect to berya.