Gittin 10B

דאי לאו דכותי חבר הוה, לא מחתים ליה מקמיה. אי הכי, אפילו שאר שטרות נמי!

as, if not for the fact that the Samaritan was one devoted to the meticulous observance of mitzvot [ḥaver], the Jew would not have allowed him to sign the document before him. Therefore, one may rely on this Samaritan in this particular case. The Talmud asks: If so, that the Mishnah is referring to that case, then even other documents should be valid as well, if a Jew signed after the Samaritan.

אלא אמרינן רווחא שבק למאן דקשיש מיניה; הכא נמי, רווחא שבק למאן דקשיש מיניה!

Rather, this is not the case with regard to other documents, as we say that the fact that the Jew signed last does not prove that this Samaritan was a ḥaver, as perhaps in signing last he was leaving space above his signature for one who was older than he is in deference to the elder, and instead, a Samaritan came and signed the document. The Talmud asks: Here too, in the case of a bill of divorce, perhaps he was leaving space above his signature for one who was his elder. Why, then, are bills of divorce and bills of manumission valid while other documents are not?

אמר רב פפא, זאת אומרת, עדי הגט אין חותמין זה בלא זה.

Rav Pappa says: That is to say, in explanation of the difference between bills of divorce and manumission and other documents, that the witnesses of a bill of divorce and a bill of manumission may not sign one without the other; rather, each witness signs in the presence of the other. A Jew would be aware that a Samaritan was signing with him, and he would not sign unless he knew that the Samaritan was a valid witness. However, with regard to other documents, witnesses are not required to sign such documents in each other’s presence. Therefore, the signature of the Jew indicates nothing about the fitness of the Samaritan witness.