Commento su Parah 8:1

שנים שהיו שומרים את השוקת – that was filled for [mixing the ashes with the water] for the lustration/sin-offering.

Two men were guarding a trough: If one of them became unclean, the water remains valid, since it is in the domain of the other. If the first became clean and the other became unclean the water is still valid since it is in the domain of the first. If both became unclean simultaneously the water becomes invalid.
If one of them did some work, the water remains valid since it is in the domain of the second. If the first stopped doing work and the other did some work, the water still remains valid since it is in the domain of the first. If both did some work at the same time the water becomes invalid.
This quite simple mishnah teaches that if two men are guarding a trough with water to be used for the red cow ritual, and one of them becomes impure or does work we can consider the water to be under the protection of the other guard, and the water remains valid. It is invalidated only if both of them are impure or do work at the same time.
The mishnah is very straightforward so there is no commentary below.

טהר – he who had become defiled was purified from his defilement.

ונטמא שני – that is his companion who was ritually pure. -

כשרים – the water that is in the trough.

עמד ושעה שני מלאכה – the first person arose from jis work, and the second did work in his place.