It occurred to me as I was at the sea thinking about the issue that Rav Shach [laws of שאלה ופיקדון] bring about a person that finds a document among his documents and he does not know whether the lender or borrower gave it to him to guard, [at the end of the first chapter of Bava Metzia] Now Rav Shach answers that it reverts to a verbal loan in which the borrower can say I paid. But I was thinking that exactly the same answers that Rav Shach had been giving before that in other cases like the third hundred might work. So in the case of the third hundred he is guarding the 300 hundred for both so both have a "Hazaka". But Rav Shach himself says about a lost object that it stays by the finder because he is not guarding it for both but only the real owner. So I was wondering why not use that same exact answer here with finding a document among his documents. It could be that both people gave it to him to guard. But we do not know that. It is possible that only one person gave it to him. Since it is doubtful we should not say divide but leave it as in fact is the law. Apparently Rav Shach did not want to use this as an answer, and I am not sure why.

[I think it is proper to add here that this is the very issue which is at the center of Rav Shach's approach in that entire section--the fact that a person that is guarding something for someone else means that person for whom he is guarding it for is considered to have a hazaka in the object. If this was not the case then you would always say המוציא מחבירו עליו הראיה. That is the only reason that in the beginning of Bava Metzia that the law is to divide--because there is a hazaka for both people.