Rambam - 1 Chapter a Day

The following rules apply when there are two paths, one impure and one pure, and a person walked down one of them, but he did not know which one he walked down. Afterwards, he came in contact with pure foods and they were eaten. He then had the ashes of the red heifer sprinkled upon himself on the third and seventh days, and then, immersed to purify himself. He then walked down the other path and came in contact with pure foods. Those foods are considered as pure.

If the foods he touched first still exist, the status of both is held in abeyance. The rationale is that one of the sets of pure food is definitely impure. If the person did not purify himself in the interim, the status of the first set is held in abeyance, while the second set should be burnt. The rationale is that these foods are certainly impure, since the person is impure because he walked down both paths.

Similar laws apply if there was a carcass of both a creeping animal and a frog in the public domain and their form was no longer recognizable and it was impossible to determine which was the creeping animal. If one touched one of the carcasses and then came in contact with pure foods which were eaten, immersed himself, touched the other one and then came in contact with pure foods, they are pure. If the foods he touched first still exist, the status of both is held in abeyance. If he did not immerse himself, the status of the first set is held in abeyance, while the second set should be burnt.

Different rules apply when there are two paths, one impure and one pure, a person walked down one of them and came in contact with pure foods and another person walked down the other and came in contact with pure foods. If they came and inquired about their status one after the other, a ruling is given to each one that he is pure. If they both came together or one came and asked about his status and that of his friend, saying: "We were two people. We walked down the two paths and we both came in contact with pure foods," they are both deemed impure and the pure foods with which they came in contact should be burnt.

Similar principles apply if they contracted impurity from a lesser source. What is implied? There were two loaves of bread, one pure and one impure. A person ate one of them and came in contact with pure foods. Another person came and ate the second and came in contact with pure foods. If they inquired about their status one after the other, they are both pure, because this is a doubt involving a question of Rabbinic Law which is one of the doubtful situations that our Sages ruled were pure, as we explained. If they both asked together or one asked about his status and that of his friend, they are both considered impure because of the doubt and the pure foods should be burnt, for certainly, one of them is impure. Even if there is one impure loaf mixed with 100 pure loaves, they are all impure and must be burnt.

When an impure loaf of bread became mixed with nine pure loaves and five people came and ate five loaves and five others came and ate the five remaining, the ones who came first are deemed impure, because they have no one else to hold accountable. The five last men are pure, because they can hold the first five accountable.

When there are two paths, one impure and one pure, and each of two people, one pure and one impure - or even one whose status was being held in abeyance - walked down one of these paths, the one who is pure can place the onus on the one who is impure or whose status is held in abeyance. We say: The person who was pure walked down the pure path and his status is still considered as pure and the one whose status was held in abeyance walked down the impure path. This applies even if they inquired about their status at the same time.