עַל־פִּ֣י שְׁנַ֣יִם עֵדִ֗ים א֛וֹ שְׁלֹשָׁ֥ה עֵדִ֖ים יוּמַ֣ת הַמֵּ֑ת לֹ֣א יוּמַ֔ת עַל־פִּ֖י עֵ֥ד אֶחָֽד׃
A person shall be put to death only on the testimony of two or more witnesses; no one shall be put to death on the testimony of a single witness.—
Torah Temimah points to Sanhedrin (continuation of the discussion of combined testimony).
תניא, לא יומת על פי עד אחד, להביא שנים שרואין אותו, אחד מחלון זה ואחד מחלון זה, ואין רואין זה את זה שאין מצטרפין, ולא עוד אלא אפילו בזה אחר זה בחלון אחד אין מצטרפין.
English: It was taught: “He shall not be put to death on the testimony of a single witness” — this includes two who see him, one from this window and one from that window, and they do not see one another: they do not combine. And not only that, but even if they see one after the other from the same window they do not combine.
In capital cases the witnesses must observe the event together as a single unit; sequential or spatially separated observation does not constitute valid combined testimony.
Derash on the verse (Bavli) — not a formal named middah of the 13. The phrase “a single witness” is expanded to cover any testimony that fails to form a unified pair.
Two witnesses who did not see the capital act together (or even sequentially from the same vantage point) cannot combine to impose the death penalty.