Comentário sobre Pará 7:2

ועשה מלאכה בידו אחת – the filling/drawing water is invalid for reason of the [extrinsic] work.

One who filled the water with one hand and did some other work with the other hand, or filled the water for himself and for another man, or filled two jars at the same time, the water of both is invalid, for work causes invalidity whether one acts for oneself or for another person. In all of these cases a person is engaged in some sort of work while he is occupied with filling water for the red cow ritual. Either he works with one hand while filling with the other, or fills for himself and another person at the same time, or fills two at the same time. In all cases, all of the water is invalid for as we have learned, one can't perform work and fill the water at the same time. The general rule is that when one is filling the water and he at the same time does work, the water is invalid not just for him, but even if he is doing it on behalf of another person. This is not true when it comes to mixing, as we saw in mishnah one.

לו ולאחר – (etc. at the same time).

שניהם פסולים – for since that for two people he is filling water, it is considered that one does work for his fellow.

בין לו בין לאחר – that even though the water is not his, he has invalidated them with the [extrinsic] work/labor that he did at the time of the drawing of the water.