Yoma, Chapter Seven, Mishnah Two
This mishnah notes that those who were in the Temple courtyard watching the priest reading the verses was too far from the bull and goat which were being burned outside of Jerusalem.
He who sees the high priest when he reads does not see the bull and the goat that are being burned, and he that sees the bull and the goat that are being burned does not see the high priest when he reads, not because he was not permitted but because the distance apart was great and both rites were performed at the same time.
The mishnah notes that a person is allowed leave the bull and goat and go and listen to the high priest. The only thing that prevents this is that it is not practicalthey were both occurring in different places but at the same time. We might have thought that it would be forbidden to go from one to the other because a person should generally not abandon performing one mitzvah in order to go perform another. However, in this case this would not be forbidden because those watching the mitzvah being performed are not in the same category as those actually performing it.