Torah Temimah cites Yerushalmi Sanhedrin 3:9.
A single witness to whom they said “you are accepted by us as two” — might he be liable? Scripture says “and he is a witness” — one who is fit to give Torah-level testimony.
The Yerushalmi excludes party-accepted single-witness arrangements from shevu’as ha-eidus. Only a witness who is fit for Torah-standard testimony falls under the verse’s liability.