לזכות חיה אביבה בת שיינדל מרים לברכה והצלחה וה' ימלא כל משאלות לבה לטובה!
The gemara says that if there are שני שבילין - two paths, A and B, and Reuven walked on one of the paths and Shimon walked on the other one and we KNOW that there is tumah on one of the paths but not on the other - when each person comes before the Beis Din to ask what his status is we tell him that he is tahor because we establish him on his chezkas kashrus [even though it is impossible that both are tahor].
This can be contrasted with the gemara [Bava Basra 31] that discusses a machlokes about two sets of witnesses who contradict each other. Rav Chisda holds that each group is pasul from here on in. Since one set of witnesses MUST be lying [and we don't know which], both sets lose their chezkas kashrus.
Why don't we say that each set retains their chazkas kashrus [even though one of them must be lying] as we do in the שני שבילין case [where they remain pure even though one of the people must be tamei]???
