Parah, Chapter 7, Mishnah 11. This chapter is built around one central principle: while a person is occupied with the mei chatas (drawing the water, carrying it, and sanctifying it with the ashes of the parah adumah), he may not turn aside to do a melachah. Any labor that does not serve the needs of the chatas ruins the water. Only an act that is itself part of the service of the water, or that makes the drawing possible, leaves the water fit. Our Mishnah applies this to a very common scene: two men standing together at the water, each one helping the other.
Here are the words of the Mishnah, clause by clause. "Shenayim shehayu memal'im lechatas": a pair of men were engaged in filling water for the chatas. "Vehigbihu zeh al zeh": they hoisted the jugs up for one another. "Venatal zeh lazeh kotzo": or one of them pulled a thorn out of the other's flesh. Now the ruling. "Bekidush echad kasher": when only a single sanctification is involved, the water remains valid. "Bishnei kidushin pasul": when two separate sanctifications are involved, it is disqualified. And then a dissenting opinion. "Rabbi Yosi omer", Rabbi Yosi maintains: "af bishnei kidushin kasher", that even where there are two sanctifications the water stays fit, "im hisnu beineihen", so long as the two of them had worked out such an understanding beforehand.
One kidush: the help is part of the drawing
Picture the first case. Both men are filling for one sanctification, meaning that the contents of both jugs will be emptied into a single vessel, and it is in that one vessel that the ashes will be placed. Since everything is heading toward one sanctification, the two acts of drawing are really one act of drawing. When this one lifts his fellow's jug onto his shoulder, or removes a thorn that is holding him up, he is not doing an outside labor at all: he is attending to the very drawing in which he himself is a partner. The help serves the chatas, and the water remains kasher.
Two kidushin: helping his fellow is a foreign labor
Now change one detail. Each man drew his own water, and each will sanctify his own water separately for himself. Here the man who lifts his fellow's jug, or who takes the thorn out of his fellow's flesh, has done something that has nothing to do with his own chatas water. It is a labor performed for someone else's needs, and by that act he has invalidated his own water. The same applies in reverse to his fellow. Two sanctifications means two separate undertakings, and what is service for one is a melachah for the other, so the Mishnah rules pasul.
Rabbi Yosi: a stipulation ties the two together
Rabbi Yosi is not satisfied that the case must end in disqualification. If the two men made an agreement beforehand that each one would assist the other in his drawing, then even with two sanctifications the water stays fit. The reason is simple and very human: once such an arrangement exists, a man's help to his companion is also help to himself. If he refuses to lift his fellow's jug, his fellow will not lift his. His own drawing depends on that mutual assistance, so lending a hand is genuinely part of the service of his own chatas water, and not a labor foreign to it.
The Mishnah thus draws a fine but clear line. What counts is not how strenuous the act is, nor whether it looks like ordinary work, but whether it belongs to the drawing of the water. Lifting a jug can be perfectly fit in one arrangement and ruinous in another, and a prior agreement between two people can transform an act of kindness for a friend into an act performed for one's own chatas.