Sforno on Exodus 23:1/ספורנו על שמות כ״ג:א׳

1לא תשא...אל תשת ידך עם רשע. To sign with such people on the same document. According to Sanhedrin 23 the inhabitants of Jerusalem would not put their signature on any document unless they had satisfied themselves as to the integrity of co-signers on such documents.

2להיות עד חמס, to be the sole signatory seeing that the signature of wicked people is not worth anything. The result might be that the judge will confiscate money from the defendant based on a document with only one signature, something inadmissible under Jewish law.