Toharot, Chapter Five, Mishnah Five
1) If there were two paths, the one unclean and the other clean, and a man walked by one of them and then prepared clean food, and subsequently another man came and walked by the second path and then prepared clean foods:
2) Rabbi Judah rules: if each by himself asked for a ruling they are both to be declared clean. But if they asked for a ruling simultaneously, both are to be declared unclean.
3) Rabbi Yose ruled: in either case they are both unclean.
Section one: The difference between this case and that in the previous two mishnayot is that here we have two people, one of whom is definitely impure. The food prepared by one of them must be impure, although the other food must be pure.
Section two: Rabbi Judah says that if each asked a sage for a ruling individually, then each is considered separately and all of the food is pure. This is because we invoke the normal rule, that cases of doubtful impurity are pure. However, if both ask together then we must treat this as if one person walked down both paths without immersing in between. One must be impure, and therefore both are regarded as impure.
Section three: Rabbi Yose rules more stringently and says that since one is definitely unclean, both must be treated as unclean.