Dear Rabbi Kornfeld,
Why does the mishna have to say that if either the son or the father are found posul the other can join with someone else? Isn't that the whole purpose in both going, regardless of whether one becomes posul or not?
Gershon
The point of the Mishnah is not only to teach that if the testimony of one
of the witnesses is found to be invalid, that the remaining witness can
join with another one. As you said, that is true for any pair of witnesses
who come to give testimony about the new moon. Rather, the point of the
Mishnah is also to teach that a father and son can go to Beis Din
*l'Chatchilah* to testify, even though already know that they cannot join together (even if each one's testimony is absolutely valid), and even if it means being Mechalel Shabbos to get there (as TOSFOS says later, 22b, DH v'Chad).
Y. Shaw