Before us is Mishnah 2 in chapter 7 of Maseches Yoma, in which the Mishnah establishes that a person cannot see with his own eyes both of the events that take place during the Yom Kippur service.
The words of the Mishnah:
"Haro'eh kohen gadol k'shehu korei - eino ro'eh par v'sa'ir hanisrafin; v'haro'eh par v'sa'ir hanisrafin - eino ro'eh kohen gadol k'shehu korei" - one who watches the Kohen Gadol at the time that he reads from the Torah cannot see the bull and the goat that are burned outside Yerushalayim; and one who watches the burning of the bull and the goat does not see the Kohen Gadol at the time of his reading.
What is the reason for this?
This is not a prohibition against seeing both, nor is there any flaw in a person being involved in both. It is simply a practical impossibility, and it rests on two factors:
The distance: a great distance separated the two places, since the bull and the goat are burned outside Yerushalayim.
The simultaneity: the two services take place at one and the same time, and a person cannot be in two places at once.
In summary: the Mishnah is not teaching a halachah of prohibition here, but stating a fact: the distance between the place where the Kohen Gadol reads and the place where the bull and the goat are burned, combined with the fact that both services are performed at the same hour, prevents an onlooker from witnessing both of them.