Rashi on Deuteronomy 19:18/רש"י על דברים י״ט:י״ח

1ודרשו השפטים היטב AND THE JUDGES SHALL DILIGENTLY ENQUIRE concerning the statement of those who assert them (the first witnesses) to be “plotting witnesses”, in that they investigate and crossexamine those who assert them to be “plotting witnesses” by diligent enquiry and scrutiny.

2והנה עד שקר העד AND, BEHOLD, IF THE WITNESS BE A FALSE WITNESS — Wherever עד is written, [except if the numeral אחד, “one” is added (cf. Rashi on v. 15)], Scripture is speaking of two witnesses (Sanhedrin 30a).