The Al HaDaf from the previous dafyomi cycle quotes the shev shmaisa regarding a case in which half a cow had been sold at the time that one of seven stomachs was found to be a treifa. Due to the logic of our gemara -- hamachshir bah machshir b'vitah -- once beis din poskens that the sold half (parush) has a din of rov, making it kosher, then the remaining half of the cow (despite being kavua) must also be declared kosher.
Isn't the gamara's case one of defining the status of one item based on the other's chezkas kashrus, while the shev shmaisa's case is one of defining the status of one item based on the other's rov? The sold half of the cow should not have a chazakah, since it was forbidden before shechita (ever min hachai or eino shachut). Can any logic based on chazakah be automatically transferred to rov?
M. Diamond
As you pointed out we do not automatically transfer all the Dinim of Chazakah to Rov. However in this case, the Shev Shmaiatza says that it is a Kal va'Chomer.
His logic is as follows. Chazakah does not actually tell me what happened; it may be equally likely that the opposite of the Chazakah is true however the Torah says that when in doubt, rely on the Chazakah. Nevertheless, we say that once we rely on the Chazakah we rely on it totally. If so, regarding Rov, which is a Birur - i.e. it does tell us what probably happened -- when we rely on it to clarify part of the situation, we should certainly rely on for all other aspects of the case as well.
Gut Yom Tov
Dov Zupnik