Halacha

הלכה א
עֵדֵי נְפָשׁוֹת צְרִיכִין שֶׁיִּהְיוּ שְׁנֵיהֶם רוֹאִים אֶת הָעוֹשֶׂה עֲבֵרָה כְּאֶחָד. וּצְרִיכִין לְהָעִיד כְּאֶחָד וּבְבֵית דִּין אֶחָד. אֲבָל דִּינֵי מָמוֹנוֹת אֵין צְרִיכִין לְכָךְ. כֵּיצַד. הָיָה אֶחָד רוֹאֵהוּ מֵחַלּוֹן זֶה כְּשֶׁעָבַר הָעֲבֵרָה וְהָעֵד הָאַחֵר רוֹאֵהוּ מֵחַלּוֹן אַחֵר. אִם הָיוּ שְׁנֵי הָעֵדִים רוֹאִין זֶה אֶת זֶה מִצְטָרְפִין וְאִם לָאו אֵין מִצְטָרְפִין. הָיָה זֶה הַמַּתְרֶה בּוֹ רוֹאֶה הָעֵדִים וְהָעֵדִים רוֹאִין אוֹתוֹ אַף עַל פִּי שֶׁאֵין רוֹאִין זֶה אֶת זֶה הַמַּתְרֶה מְצָרְפָן. הָיוּ שְׁנֵי הָעֵדִים בְּבַיִת אֶחָד וְהוֹצִיא אֶחָד מֵהֶן רֹאשׁוֹ מִן הַחַלּוֹן. וְרָאָהוּ זֶה שֶׁעוֹשֶׂה מְלָאכָה בְּשַׁבָּת וְאֶחָד מַתְרֶה בּוֹ. וְהִכְנִיס רֹאשׁוֹ וְחָזַר הָעֵד הַשֵּׁנִי וְהוֹצִיא רֹאשׁוֹ מֵאוֹתוֹ הַחַלּוֹן וְרָאָהוּ. אֵין מִצְטָרְפִין עַד שֶׁיִּרְאוּ שְׁנֵיהֶם כְּאֶחָד. הָיוּ שְׁנֵי עֵדִים רוֹאִין אוֹתוֹ מֵחַלּוֹן זֶה וּשְׁנֵי עֵדִים רוֹאִין אוֹתוֹ מֵחַלּוֹן אַחֵר וְאֶחָד מַתְרֶה בּוֹ בָּאֶמְצַע. בִּזְמַן שֶׁמִּקְצָתָן רוֹאִין אֵלּוּ אֶת אֵלּוּ הֲרֵי זוֹ עֵדוּת אַחַת וְאִם לֹא הָיוּ רוֹאִין אֵלּוּ אֶת אֵלּוּ וְלֹא צֵרֵף אוֹתָן הַמַּתְרֶה הֲרֵי אֵלּוּ שְׁתֵּי עֵדוּיוֹת. לְפִיכָךְ אִם נִמְצֵאת כַּת אַחַת מֵהֶן זוֹמְמִין הוּא וְהֵן נֶהֱרָגִין שֶׁהֲרֵי הוּא נֶהֱרָג בְּעֵדוּת הַכַּת הַשְּׁנִיָּה:
כסף משנה
1.
Both witnesses in cases involving capital punishment must see the person committing the transgression at the same time. They must deliver their testimony together, in the same court. These requirements do not apply with regard to cases involving financial matters.
What is implied? If while looking from one window, a witness saw the person commit the transgression and the other witness saw him from the other window, their testimonies can be combined if they see each other. If they cannot see each other, their testimonies cannot be combined. If a person who administered the warning sees the witnesses and the witnesses see him, because of the person administering the warning, their testimony is combined even though they do not see each other.
If they do not see the transgression at the same time, their testimony is not combined. For example, the two witnesses were in one house and one stuck his head out of the window and saw a person perform a forbidden labor on the Sabbath and another person issue a warning. He then thrust in his head and the other witness stuck his head out of the same window and saw the person commit the transgression. Their testimonies cannot be combined unless they both see the transgression at the same time.
The following laws apply when two witnesses see the transgressor from one window, two other witnesses see him from another window, and there is a person who gives a warning in between. If some of them see each other, they are considered as one group of witnesses. If they do not see each other and the person giving the warning does not include them together, they are considered as two groups of witnesses. Therefore if one group are discovered to be zomamim, the transgressor and the witnesses are executed. For the transgressor is executed on the basis of the testimony of the second group of witnesses.

הלכה ב
אֲבָל בְּדִינֵי מָמוֹנוֹת אַף עַל פִּי שֶׁלֹּא רָאוּ אֵלּוּ אֶת אֵלּוּ עֵדוּתָן מִצְטָרֶפֶת. כֵּיצַד. אָמַר הָאֶחָד בְּפָנַי הִלְוָהוּ בְּיוֹם פְּלוֹנִי אוֹ בְּפָנַי הוֹדָה לוֹ. וְאָמַר הָעֵד הַשֵּׁנִי וְכֵן אֲנִי מֵעִיד שֶׁהִלְוָהוּ בְּפָנַי אוֹ הוֹדָה בְּיוֹם אַחֵר. הֲרֵי אֵלּוּ מִצְטָרְפִין:
כסף משנה
2.
With regard to cases involving financial matters, by contrast, even though they did not see each other, their testimony can be combined.
What is implied? One witness said: "In my presence, he lent money him on this-and-this day" or "In my presence, he acknowledged a debt," and the second witness says: "I also testify that he lent him money" or "...acknowledged a debt" on a different day, their testimony can be combined.

הלכה ג
וְכֵן אִם אָמַר הָאֶחָד בְּפָנַי הִלְוָהוּ וְהַשֵּׁנִי אוֹמֵר בְּפָנַי הוֹדָה לוֹ. אוֹ שֶׁאָמַר הָרִאשׁוֹן בְּפָנַי הוֹדָה וְהַשֵּׁנִי שֶׁהֵעִיד אַחַר זְמַן אָמַר בְּפָנַי הִלְוָהוּ. הֲרֵי אֵלּוּ מִצְטָרְפִין:
כסף משנה
3.
Similarly, if one witness states: "He gave a loan in my presence," and the other said: "He acknowledged a debt in my presence," or the first said: "He acknowledged a debt in my presence," and the other testified afterwards, saying: "He gave a loan in my presence," their testimony can be combined.

הלכה ד
וְכֵן בְּעֵת שֶׁמְּעִידִין בְּבֵית דִּין יָבוֹא אֶחָד וְשׁוֹמְעִין דְּבָרָיו הַיּוֹם וּכְשֶׁיָּבוֹא הָעֵד הַשֵּׁנִי לְאַחַר זְמַן שׁוֹמְעִין דְּבָרָיו וּמִצְטָרְפִין זֶה לָזֶה וּמוֹצִיאִין בָּהֶן הַמָּמוֹן:
כסף משנה
4.
Similar concepts apply with regard to the time of their testimony in court. One may come on one day and the court will hear his testimony and the other may come on a later date and have his testimony heard. The testimonies may be combined and money expropriated on this basis.

הלכה ה
וְכֵן אִם הָיָה עֵד אֶחָד בִּכְתָב וְאֶחָד עַל פֶּה מִצְטָרְפִין. וְאִם אָמַר זֶה שֶׁלֹּא כָּתַב עֵדוּתוֹ אֲנִי קָנִיתִי מִיָּדוֹ עַל דָּבָר זֶה וְלֹא בָּא הַמַּלְוֶה הַזֶּה וְלֹא שָׁאַל מִמֶּנִּי לִכְתֹּב שְׁנֵיהֶם מִצְטָרְפִין לַעֲשׂוֹת הַמִּלְוֶה בַּשְּׁטָר וְאֵינוֹ יָכוֹל לוֹמַר פָּרַעְתִּי:
כסף משנה
5.
Similarly, if the testimony of one witness was recorded in a legal document and the other testified orally, their testimony may be combined. If the witness who did not record his testimony states: "I entered into an act of contract with him concerning this manner, but the lender did not come and ask me to record my testimony in a legal document," the two can join together to give the claim the status of a loan backed by a promissory note. The borrower may not claim: "I repaid the debt."

הלכה ו
הֵעִיד הָאֶחָד בְּבֵית דִּין זֶה וְהָעֵד הַשֵּׁנִי בְּבֵית דִּין אַחֵר יָבוֹא בֵּית דִּין אֵצֶל בֵּית דִּין וְיִצְטָרְפוּ עֵדוּתָן. וְכֵן אִם הֵעִידוּ שְׁנֵי הָעֵדִים בְּבֵית דִּין זֶה וְחָזְרוּ וְהֵעִידוּ בְּבֵית דִּין אַחֵר יָבוֹא אֶחָד מִכָּל בֵּית דִּין וְיִצְטָרְפוּ. אֲבָל הָעֵד עִם הַדַּיָּן שֶׁהֵעִידוּ שְׁנֵי הָעֵדִים בְּפָנָיו אֵין מִצְטָרְפִין:
כסף משנה
6.
The following laws apply in cases involving financial matters. If one witness delivered testimony in one court and the other witness delivered testimony in a second court, the two courts should come together and combine the testimonies. Similarly, if two witnesses delivered testimony in one court and then delivered testimony in another court, a member of either court can join together with a member of the other court. The statements of a witness and a judge before whom two witnesses testified may not be combined.

הלכה ז
אַף עַל פִּי שֶׁמִּצְטָרְפִין הָעֵדוּת בְּדִינֵי מָמוֹנוֹת צָרִיךְ שֶׁיָּעִיד כָּל אֶחָד מִשְּׁנֵיהֶם בְּכָל דָּבָר כְּמוֹ שֶׁבֵּאַרְנוּ. אֲבָל אִם הֵעִיד עֵד אֶחָד בְּמִקְצָת דָּבָר וְהֵעִיד הַשֵּׁנִי בְּמִקְצָתוֹ אֵין מְקַיְּמִין הַדָּבָר מֵעֵדוּת שְׁנֵיהֶם. שֶׁנֶּאֱמַר (משנה סוטה ו ג) "עַל פִּי שְׁנַיִם עֵדִים יָקוּם דָּבָר". כֵּיצַד. זֶה אוֹמֵר פְּלוֹנִי אָכַל שָׂדֶה זוֹ שָׁנָה פְּלוֹנִית וְזֶה הֵעִיד שֶׁאֲכָלָהּ שָׁנָה שְׁנִיָּה וְזֶה הֵעִיד שֶׁאֲכָלָהּ שָׁנָה שְׁלִישִׁית. אֵין מִצְטָרְפִין עֵדוּת שְׁלָשְׁתָּן וְאוֹמְרִים הֲרֵי אֲכָלָהּ שָׁלֹשׁ שָׁנִים שֶׁכָּל אֶחָד וְאֶחָד הֵעִיד בְּמִקְצָת הַדָּבָר. וְכֵן אִם הֵעִיד זֶה אֲנִי רָאִיתִי שַׂעֲרָה אַחַת בְּצַד יְמִינוֹ שֶׁל זֶה וְזֶה אוֹמֵר אֲנִי רָאִיתִי שַׂעֲרָה אַחַת בְּצַד שְׂמֹאלוֹ שֶׁל זֶה בְּאוֹתוֹ הַיּוֹם. אֵין מִצְטָרְפִין דִּבְרֵי שְׁנֵיהֶם כְּדֵי שֶׁנֹּאמַר הֲרֵי הֵעִידוּ שְׁנֵיהֶם שֶׁהָיָה זֶה גָּדוֹל בְּיוֹם פְּלוֹנִי לְפִי שֶׁכָּל אֶחָד מֵהֶם לֹא הֵעִיד אֶלָּא בְּמִקְצָת הַסִּימָנִין. אֲפִלּוּ הֵעִידוּ שְׁנַיִם בְּשַׂעֲרָה אַחַת וְהָעִידוּ שְׁנַיִם בְּשַׂעֲרָה אַחֶרֶת שֶׁהֲרֵי כָּל כַּת מֵהֶם הֵעִידָה עַל חֲצִי דָּבָר וְאֵין זוֹ עֵדוּת. אֲבָל אִם הֵעִידָה הָאַחַת שֶׁרָאֲתָה שְׁתֵּי שְׂעָרוֹת בְּצַד יָמִין וְהֵעִידָה הַשְּׁנִיָּה שֶׁרָאֲתָה שְׁתֵּי שְׂעָרוֹת בְּצַד שְׂמֹאל מִצְטָרְפִין. וְכֵן כָּל כַּיּוֹצֵא בּוֹ:
כסף משנה
7.
Although testimony of two witnesses may be combined in matters of financial law, each of the witnesses must deliver testimony concerning an entire matter, as we explained. If, by contrast, one witness testifies concerning a portion of a matter and the other witness testifies concerning another portion of the matter, we do not establish the matter on the basis of their testimony, as indicated by Deuteronomy 19:15: "According to the testimony of two witnesses shall the matter be established."
What is implied? One witness testifies that a person benefited from a field one year, another testifies that he benefited in the following year, and a third testifies that he benefited in the third year, the testimonies of the three cannot be linked together to say that he benefited for three years. For each of them testified only about a portion of the matter.
Similarly, if one witness testifies: "I saw one hair on the person's right side," and another witness testifies: "I saw one hair on the person's left side," their testimonies are not linked together so that we can say that two people testified that the person concerned manifested signs of physical maturity on that particular day. For each of them testified only about a portion of the physical signs required. Even if two witnesses testified that they saw one hair and two other witnesses testified that they saw another hair, their testimony is of no consequence. Since they both testified about only half the matter, this is not acceptable testimony.
If, however, one witness testified that he saw two hairs on the person's right side and another witness testified that he saw two hairs on the person's left side, their testimony can be linked together. Similar concepts apply in all analogous situations.

שופטים הלכות עדות פרק ד
Shoftim Edus Chapter 4