I was on y way back from the sea shore and it suddenly occurred to me what Tosphot is saying. It is this. Tosphot on page 18 says we believe the witnesses when they say they were not valid witnesses because the doc is thus lacking the requirement of establishing its validity. On page 19 Tos says we do not belie them when they say the signatures on the doc are theirs and are valid but there never was any loan. We believe them because they admit the document was written and signed according to law. What Tosphot means is this. We require them only to testify about their signatures, nothing else. {They need to remember the loan but that is not what they are testifying about.} Thus, in the Mishna they are believed because they are asked if the signatures on the doc are theirs, and they say “Yes, but we were not valid witnesses” and thus they are believed and the doc lacks validation. In the case on page 19, they are not asked about the loan. They are asked if they signed this doc and they say yes, and that the signatures are valid, but they also add that there was no loan, and that is not what they are testifying about. Thus, their statement contradicts the valid doc and so it is a case of two against two.---------------------------------It occurred to me what תוספות is saying. It is this. תוספות on page 18 says we believe the witnesses when they say they were not valid witnesses because the שטר is thus lacking the requirement of establishing its validity. On דף י''ט תוספות, says we do not believe them when they say the signatures on the שטר are theirs and are valid, but there never was any loan. We believe them because they admit the שטר was written and signed according to law. What תוספות means is this. We require them only to testify about their signatures, nothing else. {They need to remember the loan (according to the רמב''ם but not according to the ראב''ד), but that is not what they are testifying about.} Thus, in the משנה, they are believed because they are asked if the signatures on the שטר are theirs, and they say “Yes, but we were not valid witnesses,” and thus they are believed, and the שטר lacks validation. In the case on דף י''ט, they are not asked about the loan. They are asked if they signed this שטר , and they say yes, and that the signatures are valid, but they also add that there was no loan, and that is not what they are testifying about. Thus, their statement contradicts the valid שטר and so it is a case of two against two. תרי ותרי-שנים על השטר כנגד שניים בעל פה