“Two people who borrowed together are jointly responsible and mutual guarantors for each other,” answered Rabbi Dayan. “Therefore, each of you should pay an equal share in the loss. If one is unable to pay his share, the others remain liable as guarantors for that amount. They are entitled to collect reimbursement from him later, when he is able to pay.” (C.M. 77:1; Machane Ephraim, Shomrim #27)
“What if one person was negligent?” asked Chaim. “For example, if I had forgotten to lock the doors?”
“Then you would be liable for the full amount,” said Rabbi Dayan. “If you were unable to pay, the other borrowers would still remain liable as guarantors but would be entitled to reimbursement from the negligent party when you are able to pay. (See Shach 77:1; Nesivos 77:1; Pischei Choshen, Pikadon 1:16 33)