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 (same discussion).
מאי אחד, אילימא עד אחד ממש, מרישא שמעינן לה, על פי שנים עדים, אלא מאי אחד – אחד אחד, מכאן לעדות מיוחדת שהיא פסולה.
English: What is meant by “one”? If you say a single witness in the literal sense, we already know that from the beginning of the verse (“on the testimony of two witnesses”). Rather, what is “one”? One-and-one — from here [we learn] that disjoint testimony is invalid.
The word “echad” is read as “one by one,” teaching that testimony given separately by each witness does not combine for capital cases.
Derash on the verse (Bavli) — not a formal named middah of the 13. The seemingly redundant “one” is taken as a technical term for sequential or disjoint testimony.
Disjoint (miyuchedet) testimony — each witness observing or testifying separately — is invalid for capital punishment.