Gemara

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.

30b (Rav Yehudah): If the witnesses contradicted each other about Bedikos (matters not essential to the testimony), the testimony is valid for monetary cases.

32a (Mishnah #1): Drishah v'Chakirah (interrogation of the witnesses) applies to monetary and capital cases alike - "Mishpat Echad Yihyeh Lachem".

Contradiction (Mishnah #2): Postdated loan documents are Kesherim (even though Drishah v'Chakirah does not apply to them)!

Answer #1 (R. Chanina): Mid'Oraisa, Drishah v'Chakirah applies to monetary and capital cases alike. Chachamim enacted that monetary cases do not require Drishah v'Chakirah due to Ne'ilas Delet (lest potential lenders fear that the witnesses will not pass Drishah v'Chakirah).

Answer #2 (Rava): Mishnah #1 discusses fines. Ne'ilas Delet applies only to admissions and loans. Drishah v'Chakirah was abolished only for them.

Answer #3 (Rav Papa): Both discuss admissions and loans. Mishnah #1 discusses a Din Merumeh (Beis Din senses that the claim is false), Mishnah #2 is a normal case.

Bava Kama 75b: Testimony that cannot be Huzam is invalid, i.e. when the witnesses do not know at what time they saw the testimony.

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): If so, they should be Kosher for documents, in which Hazamah does not apply! The Bavli rejected this reasoning. Rather, a Gezeras ha'Kasuv disqualifies relatives. The Ri mi'Gash and Ba'al ha'Ma'or say that just like witnesses can validate their relatives' signatures, also judges can. Why did the Rif bring the Yerushalmi, after the Bavli rejected it? Perhaps it is to teach that the witnesses may not be related to the judges. This is not explicit in the Bavli, but it follows from the Bavli's reasoning. Presumably, this applies even to validation of documents. However, the Rashba allows this, since Hazamah applies only if the ones who recognize the signatures say 'they signed the document in front of us', which is very rare. We are not concerned for this regarding validation of documents, which is mid'Rabanan. I say that the Bavli concluded that this does not depend on Hazamah.

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 Malveh 17:9: 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 for Drishah v'Chakirah, lest people be deterred from lending. Therefore, postdated documents are Kosher, even though Iy Efshar Lehazimah.

Ohr Some'ach (Hilchos Edus 4:1): I am unsure about one who killed without witnesses, i.e. without warning or the witnesses did not see each other (in which case we put the murderer in Kipah and cause him to die). Does Hazamah apply to put the witnesses in Kipah? Presumably, we do not. Even though the testimony is Iy Efshar Lehazimah, we can answer like Tosfos (Makos 2a), that lashes fulfill Ka'asher Zomam, or that testimony she'Iy Efshar Lehazimah is Kosher for monetary (i.e. all non-capital) cases, like the Rambam connotes in Hilchos Rotze'ach (2:9).

Question (Tosfos Makos 2a DH Me'idin): A witness cannot become a Ben Grushah (a Chalal) through Hazamah. This is Iy Efshar Lehazimah!

Answers (Tosfos): Being a Chalal entails only Lavim. Since the witnesses are lashed, this is called Efshar Lehazimah. Alternatively, the Torah does not require Efshar Lehazimah for such 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.

Beis Yosef (DH u'Mah she'Chosav Rabeinu v'Af and DH Kosav Ba'al): Rav Yehudah taught that if the witnesses contradicted each other about Bedikos, the testimony is valid for monetary cases. This implies that a contradiction about Drishos v'Chakiros invalidates the testimony. Sefer ha'Terumos (29:4:1) says that this is Kosher. Rav Yehudah mentioned Bedikos (for this is the usual case,) for we do not do Drishah v'Chakirah for monetary cases. The Rambam disagrees. Chachmei Lunil sided with the Rambam, but for a different reason.

Hagahos Tur ha'Shalem (26): The Rambam holds that the testimony of both is disqualified. Chachmei Lunil hold that the testimony of the first is disqualified, and the second obligates only a Shevu'ah.

Nesivos ha'Mishpat (Bi'urim 2 DH v'Hinei): If witnesses testified that Levi borrowed from David 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. In capital cases, we can execute only if we know due to which witnesses he is killed, so it is Efshar Lehazimah. Similarly, if two distinct pairs of witnesses testified that Levi is Chayav Misah, and Beis Din knows that one of the pairs is Pasul, but does not know which, they cannot kill Levi, for this cannot be Huzam. This proves that we do not require Efshar Lehazimah for monetary cases, for when witnesses argue about how much Levi owes, he must pay the smaller amount (Sanhedrin 31a)

Shulchan Aruch (33:17): 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): The Mordechai and Nimukei Yosef (and Ba'al ha'Itur and Rambam) require Efshar Lehazimah even for monetary cases! They mean 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. If witnesses contradicted each other about Bedikos, the testimony is valid for monetary cases (30b). This implies that a contradiction about Chakiros is Mevatel the testimony. Chakiros are different, for we need Efshar Lehazimah.

Beis Yosef (EH 11 DH v'Chosav Nimukei): The Ramban (Yevamos 122a DH Ha) says that nowadays we consider Edus Ishah (testimony that a man died, to permit his widow) to be monetary testimony (so it does not need Drishah v'Chakirah), for most women have Kesuvos. We apply this even to an Arusah, who has no Kesuvah. The Mordechai says that witnesses of Zenus need not see together, like monetary cases. However, there is no reason to enact not to require Drishah v'Chakirah for this. The Rashba (3:111) says that Gitin and Kidushin do not need Drishah v'Chakirah, for now they are not capital testimony.

Noda b'Yehudah (1: EH 72 DH Shuv Motzasi): Mahara Sason says that to forbid a woman to her husband is like capital cases. We require Drishah v'Chakirah. If a witness cannot answer a question, the testimony is Batel. He learns from Sanhedrin 41a, which discusses Hazamah of witnesses who say that they only intended to forbid her. Presumably, they say the time only if the judges asked them, i.e. for we need Drishah v'Chakirah! Really, we require Drishah v'Chakirah, but not Efshar Lehazimah.