According to the Mishna on today’s daf, if a person approaches two people and admits that he stole 100 zuz from one of them, but that he does not remember from whom he stole, he will have to pay each of them 100 zuz. The Mishna rules similarly in a case where he tells two people that he received 100 zuz from one of their fathers to watch, but he doesn’t remember whose father gave him the money.
The Mishna’s explanation for this is she-hodah mi-pi atzmo – that he admitted his obligation on his own. That is to say, since he desires to make amends, this is the only way to be sure that he is repaying his debt. According to the letter of the law, since he only owes one of them money, and neither of them has a claim against him, he is only obligated to pay 100 zuz one time, and the two people should split the money (see the Rambam in his Mishneh Torah Hilkhot Gezeila VaAveda 4:10). It should be noted that if he was responding to claims made by the two people, each of whom said that he owed them 100 zuz, if they both swear that he owes them money he will have to pay each of them. This is true because either as a thief or as an irresponsible guard who did not remember who gave him money to watch, he will be accountable for the loss.
The Rosh points out that when the Mishna presents the case as one of the fathers giving him money to guard, it could not have been a case where he said that one of the people themselves gave him money. In such a case, if neither he nor they remembered who gave him the money, he will not have to pay anybody, since it would be the responsibility of the person who gave him the money to remember that he did so.