chazakah vs. eidus

Last week I touched on Tosfos Nazir 57 but missed an important point. To recap: the Mishna (Nazir 57) describes a case of a witness who sees one of two nezirim become tamei and is not sure which one it was. The gemara asks why there is a safeik - since this case involves 2 nezirim + the observer, it should be considered safeik tumah in reshus harabim and both nezirim should be tahor. The gemara answers that the case must be where the observer was not immediately proximate to the two nezirim but just saw tumah thrown in their direction. Tosfos asks: If the observer is not proximate to the nezirim, then this should be a case of safeik tumah in reshus hayachid and both nezirim should be tamei? Tosfos answers that the rule of safeik tumah b’rechus hayachid tamei, which is derived from the parsha of sota, does not apply where we are forced to draw two mutually exclusive conclusions. The fact is that only one nazir became tamei. The halachic rule of safeik tumah b’reshus hayachid would force us into the counterfactual conclusion that both nezirim are tamei.

If we throw out the rule of safeik tumah b’reshus hayachid, then why not also throw out the rule of safeik tumah b’reshus harabim? What then was the gemara’s question to begin with? Tosfos answers that the rule of safeik tumah b’reshus harabim is not based on a gezeiras hakasuv from parshas sota but is based on chazaka. Each individual nazir has a chezkas tahara. Even if both chazakos mutually exclude each other, until we can prove which nazir became tamei, the chazakos still stand.

I never explained (and it did not dawn on me until Shabbos) why this distinction should be true. Why is it that the rule of safeik tumah derived from parshas sotah cannot be applied where factually there is no way both nezirim are tamei, but the rule of chazakah can be applied even though we know factually both nezirim cannot be tehorim? I'll leave this one unanswered for now...