no Yes. Assume the two signers had an agreement between themselves that they would share the cost of the rent evenly. Then assume that for some reason, (like losing a job) one could not pay his/her half and asked the other to pay the entire rent and get reimbursed later (after getting a job). If the first renter reneges on the promise of reimbursement, the other renter is allowed to sue the first one for compensation for the extra rent paid. The fact that that the reimbursement agreement is not in the lease is irrelevant, because the agreement between the two renters is a separate contract. It may be enforced according to its own terms as long as it meets the legal requirements for a valid enforceable contract under the laws of that state.