Comentario sobre Números 18:3

ושמרו משמרתך ומשמרת האהל, the Levites are to guard the Holy of Holies (structure) which in itself is your domain; they are also to guard all the other sacred locations outside the Holy of Holies.

גם הם גם אתם, "neither they nor you." According to the plain meaning the words גם הם mean "just as the Israelites who approach;" the words גם אתם mean "when you act on their account" just as in the previous verse the words about אתה ובניך.

ולא ימותו גם הם גם אתם, “so that not both you and they will die.” This is the source for the halachah that the functions of the Levites are not interchangeable, that a priest may not perform functions designated for the Levite, nor the Levite functions reserved for the priest (Erchin 11, Maimonides Hilchot Kley Hamikdash 3,10). Not only this, but even priests who have been assigned certain tasks must not exchange their duties with priests who have been assigned different priestly assignments, nor must Levites exchange duties within the framework of what is their exclusive domain. If a Levite was assigned the duty to play an instrument or participate in the songs the Levites sing, he must not instead engage in washing the hands of the priests, for instance! This is why the Torah writes (Numbers 4,49) איש איש על עבודתו, “each man to his designated task.” This is the way these words have been defined in Sifri 116. The words: “but to the holy vessels and to the Altar they shall not approach,” are a warning, whereas the words following “so that they will not die,” are the Torah’s way of spelling out the penalty for non-observance of the warning. The words גם הם, “they also,” are the source for the inadmissibility of exchanging duties either by mutual consent or willful pre-empting the duties assigned to a colleague. It occurred once that Rabbi Joshua ben Chanayah wished to assist a colleague in closing the doors and he was stopped from doing so by Rabban Yochanan who told him that seeing he was one of the singers he had not business to concern himself with the duties of those who were in charge of the gates (Erchin 11).