מֵעִיד קָרוֹב עַל כְּתַב קְרוֹבוֹ. כֵּיצַד. שְׁטָר שֶׁעֵדָיו רְאוּבֵן וְשִׁמְעוֹן וּמֵתוּ אוֹ שֶׁהָלְכוּ לִמְדִינַת הַיָּם וּבָא בְּנוֹ שֶׁל רְאוּבֵן וְאָמַר זֶה כְּתַב יָדוֹ שֶׁל אָבִי וּבָא בְּנוֹ שֶׁל שִׁמְעוֹן וְאָמַר זֶה כְּתַב יָדוֹ שֶׁל אָבִי הֲרֵי אֵלּוּ כִּשְׁנֵי עֵדִים כְּשֵׁרִים שֶׁאֵינָן קְרוֹבִים וְאִם יִצְטָרֵף עִמָּהֶם שְׁלִישִׁי וְהֵעִיד עַל כְּתַב יָדָן שֶׁל שְׁנֵיהֶם הֲרֵי נִתְקַיֵּם הַשְּׁטָר:
A relative may give testimony with regard to his relative's signature. What is implied? There was a legal document which Reuven and Shimon signed as witnesses. They died or traveled overseas. Reuven's son came and testified: "This is my father's signature," and Shimon's son came and testified: "This is my father's signature," it is as if they are two acceptable witnesses who are not related to the witnesses who have signed. If a third witness joins together with them and testifies with regard to the two signatures, the authenticity of the legal document is validated.
