Mishnayos Makkos Perek 1 Mishnah 9

In a case where there were two witnesses observing an individual violating a capital transgression from this window in a house, and two observing him from that window in a house, and one person was forewarning the transgressor in the middle between the two sets of witnesses, the halakha depends on the circumstances. In a situation where some of the witnesses observing from the two windows see each other, the testimony of all these witnesses constitutes one testimony, but if they do not see each other, the testimony of these witnesses constitutes two independent testimonies. Therefore, as two independent sets of witnesses, if one of the sets was found to be a set of conspiring witnesses, while the testimony of the other set remained valid, both he, the one accused of violating the capital transgression, and they, the conspiring witnesses, are executed, and the second set, whose testimony remained valid, is exempt. Rabbi Yosei says: Transgressors are never executed unless his two witnesses are the ones forewarning him, as it is stated: “At the mouth of two witnesses…he who is to be put to death shall die” (Deuteronomy 17:6), from which it is derived that it is from the mouths of the two witnesses that the accused must be forewarned, and forewarning issued by someone else is insufficient. Alternatively, from the phrase “at the mouth of two witnesses” one derives that the judges must hear the testimony directly from the witnesses, and the Sanhedrin will not hear testimony from the mouth of an interpreter.

הָיוּ שְׁנַיִם רוֹאִין אוֹתוֹ מֵחַלּוֹן זֶה וּשְׁנַיִם רוֹאִין אוֹתוֹ מֵחַלּוֹן זֶה וְאֶחָד מַתְרֶה בוֹ בָּאֶמְצַע, בִּזְמַן שֶׁמִּקְצָתָן רוֹאִין אֵלּוּ אֶת אֵלּוּ, הֲרֵי אֵלּוּ עֵדוּת אַחַת. וְאִם לָאו, הֲרֵי אֵלּוּ שְׁתֵּי עֵדֻיּוֹת. לְפִיכָךְ אִם נִמְצֵאת אַחַת מֵהֶן זוֹמֶמֶת, הוּא וָהֵן נֶהֱרָגִין וְהַשְּׁנִיָּה פְּטוּרָה. רַבִּי יוֹסֵי אוֹמֵר, לְעוֹלָם אֵין נֶהֱרָגִין עַד שֶׁיְּהוּ פִּי שְׁנֵי עֵדָיו מַתְרִין בּוֹ, שֶׁנֶּאֱמַר (דברים יז) עַל פִּי שְׁנַיִם עֵדִים. דָּבָר אַחֵר, עַל פִּי שְׁנַיִם עֵדִים, שֶׁלֹּא תְהֵא סַנְהֶדְרִין שׁוֹמַעַת מִפִּי הַתֻּרְגְּמָן: