Gemara

(Beraisa): If two witnesses testified that Reuven stole, and two others testified that he sold, and the witnesses about the sale were Huzmu, Reuven pays Kefel, and the Edim Zomemim pay two or three;

Sumchus says, the Edim Zomemim pay Kefel, and Reuven pays two for a Seh or three for an ox.

(Rav Acha brei d'Rav Ika): They argue about testimony that cannot be Huzam. Two witnesses said 'you stole', and Reuven answered, I stole and sold, but I did not steal in front of you, rather, in front of Ploni and Almoni, and Ploni and Almoni testified that he stole and sold.

Chachamim hold that since Ploni and Almoni cannot be Huzmu (for Reuven admitted that they saw him steal and sell), their testimony is invalid;

Sumchus says, even though they cannot be Huzmu, their testimony is valid.

We hold that testimony that cannot be Huzam is invalid, i.e. when the witnesses do not know at what time they saw the testimony. Such testimony is worthless. Here, the accused supports the testimony!

Sanhedrin 28a - Question: What is the source that two relatives cannot testify together (about someone else)?

Answer #1 (Rami bar Chama): Reasoning teaches this. Witnesses are not made Zomemim unless both are Huzmu. If relatives could testify about someone else, they could be killed due to each other's testimony!

Objection (Rava - Mishnah): If David testified about three years of Chazakah, and three brothers testified, each about one of the three years, it is like one testimony for Hazamah (they are exempt unless all of them are Huzmu).

If they will be Huzmu, the brothers will pay due to each other's testimony! Rather, they pay due to the Hazamah, not due to each other's testimony. Likewise, if relatives testified together and were Huzmu, they are killed due to the Hazamah, not due to each other's testimony!

Answer #2 (Rava): It says "u'Vanim" (and not 'Ben Al Avos' or 'Hem Al Avos') to teach that Banim (i.e. relatives) cannot testify with each other.

78a (Rava): If witnesses testified about a Treifah and they were Huzmu, they are exempt. If Treifah witnesses were Huzmu, they are killed.

Rebuttal (Rav Ashi): Also Treifah witnesses who were Huzmu are not killed, for the testimony of the Mezimim (who said that the witnesses were elsewhere) cannot be Huzam (we could not kill them for trying to kill Treifos).

Rishonim

Rif and Rosh (Sanhedrin 8b and 3:32): The Yerushalmi disqualifies witnesses related to each other or to the judges due to Hazamah (lest they be killed due to each other).

Ran (Kesuvos 12b DH Kivan): The Bavli concluded that the Pesul of relatives is not due to Hazamah. Why does the Rif rely on similar reasoning to disqualify witnesses related to the judges? Rather, since the Bavli learned from "Banim" that relatives may not testify with each other, the same applies to testifying in front of relatives. Perhaps the Rif brought the Yerushalmi only to teach that the witnesses may not be related to the judges, for this is not explicit in the Bavli, but it follows from the reasoning of the Bavli.

Nimukei Yosef (Bava Kama 32a DH Yerushalmi): The Yerushalmi says that witnesses cannot be related to the judges. A witness cannot become a judge because it is Iy Efshar Lehazimah.

Rambam (Hilchos Edus 3:3): Even though monetary cases do not require Drishah v'Chakirah, if it was done and the witnesses contradicted each other, the testimony is Batel.

Rambam (Hilchos Rotze'ach 2:9): Any testimony she'Iy Efshar Lehazimah is invalid for capital cases.

Question (Lechem Mishneh Hilchos Edus 20:7): This connotes that testimony she'Iy Efshar Lehazimah is Kosher for capital cases. Sanhedrin 78a says that Stam that it is invalid! It seems that the Ran (above) holds that this is even for monetary cases.

Answer (Shach CM 33:16): (Sanhedrin 78a disqualifies when witnesses cannot answer Chakiros or contradict each other. The Rambam rules like this in Hilchos Edus 3:3). Here (and in Hilchos Malveh below), the Rambam teaches that for monetary cases, l'Chatchilah we do not probe the witnesses, so Hazamah is not feasible.

Mishneh l'Melech (Hilchos Geneivah 3:8): Rav Acha brei d'Rav Ika explains that Chachamim hold that since Ploni and Almoni cannot be Huzmu (for Reuven admitted that they saw him steal and slaughter or sell), their testimony is invalid. Perhaps testimony she'Iy Efshar Lehazimah is Pasul only for capital cases and fines. The Rivash (266) says that if even if witnesses cannot answer the Chakiros, the testimony is Kosher for monetary cases.

Hagahos Ashri (Kesuvos 13:20): If a document does not say where it was written, or it has no date, it is Kosher. In monetary cases we are not concerned whether the testimony can be Huzam.

Mordechai (Sanhedrin 796): The Sar mi'Kutzi says that witnesses can be relatives of the judges, for they can be Huzmu in another Beis Din. Witnesses cannot become judges because it is Iy Efshar Lehazimah. If they will be Huzam, the final verdict was invalid, for the judges were Pasul. Even the opinion that does not disqualify Edim Zomemim retroactively agrees that they are disqualified for that testimony.

Poskim

Shulchan Aruch (30:2): Even though we do not require Drishah v'Chakirah for monetary cases, if the witnesses contradicted each other in Drishos v'Chakiros, their testimony is Batel.

Shach (5,16): It is Batel because it is Iy Efshar Lehazimah. This is unlike the Rivash (266), who is Machshir witnesses who cannot answer Chakiros). Bava Kama 75b says that such testimony is worthless! It is difficult to say that this refers only to fines.

Nesivos ha'Mishpat (Bi'urim 2 DH v'Hinei): If witnesses testified that Reuven borrowed from Levi in a certain time and place, and other witnesses contradicted this, but said that he lent to him at another time, he is liable, for in any case he owes for one loan. Regarding capital cases we cannot execute him, for we do not know due to which witnesses he is killed, so it is Iy Efshar Lehazimah. This proves that we do not require Efshar Lehazimah for monetary cases, for when witnesses argue about how much Reuven owes, he must pay the smaller amount (Sanhedrin 31a)

Shulchan Aruch (17): Witnesses related to the judges are Pesulim.

SMA (26): The Mordechai and Nimukei Yosef hold that only capital cases require Efshar Lehazimah. They allow witnesses who are related to the judges or are Tereifos, for monetary cases. The Rema omitted their opinion because many disagree.

Rebuttal (Shach 16): Also the Mordechai and Nimukei Yosef require Efshar Lehazimah. They merely say that presumably, the witnesses know the time and place, so we need not check if they know. Indeed, if we find that they do not know, the testimony is invalid. We must explain the Ba'al ha'Itur and Rambam similarly. They disqualify witnesses related to the lender or judges due to Iy Efshar Lehazimah, and elsewhere they say that mid'Rabanan, this is not needed for monetary cases. The Mordechai cites the Ri, who allows witnesses related to the judges. Hazamah is possible in another Beis Din (Tosfos in the name of the Ri). Sanhedrin (32a) requires Drishah v'Chakirah for monetary cases, i.e. to make Hazamah possible (41b). The enactment not to require Drishah v'Chakirah for loans and admissions was only that Beis Din need not ask, but we require Efshar Lehazimah. Chachamim would not enact that we do not need Efshar Lehazimah. If witnesses contradicted each other about Bedikos, the testimony is valid for monetary cases (Sanhedrin 30b). This implies that if they contradicted each other about Chakiros, the testimony is Batel. This is reasonable only if we need Efshar Lehazimah. If not, why is it different than Bedikos?! Likewise, if a witness could not answer a Chakirah, his testimony is Batel, for it is Iy Efshar Lehazimah.

Gra (34): The Ran says that even though the Gemara concluded that the Pesul of witnesses who are relatives is not due to Hazamah, the law is true (we require Efshar Lehazimah).