Nazir 57 - One out of two nazirs
If two nazirs (correctly, nazirim) were walking together on the road, and somebody saw that one of them became ritually impure, they have a problem on their hands. The impure one must shave and bring sacrifices to purify himself, and the pure one simply continues being a nazir. But they don't know who is who.
Here is their way out. They both continue being a nazir until the end of their term. Then they both shave, because it would be required for both pure and impure nazir. Then they take two sets of sacrifices, one for a nazir who is pure, and another, different set, for a one who is a impure, and bring them together to the Temple.
Each one of them now says: "If I am really pure, and my friend is impure, then the sacrifices of purity are mine, and the other set is his." Then they bring both sets. But this works only for a pure nazir. Now they continue for yet another thirty days, and at the end they bring just one set of sacrifices - for the nazir who is pure. Each one says: "If I was the pure one - then this sacrifice is my friend's, and previously my sacrifice was already brought. But if I was the impure one, then this sacrifice is now mine."
The Talmud then asks: why is there any doubt at all? We know how to resolve all such doubts based on the laws of the doubts of a suspected wife (next Tractate). That is, if the suspected people hid, the doubt is confirmed. And if they they were in a public place, there is no doubt at all. Here they did not hide from the one who saw them!? - He was far, so it is as if they were hiding.
Art: Portrait of a Clean-Shaven Young Man by Lucas The Elder Cranach