When two or more renters sign the same rental agreement or lease, they are co-tenants. All co-tenants, regardless of agreements they make among themselves, are legally responsible to the landlord for the entire amount of the rent.
Ted and Karen sign a month-to-month rental agreement for an $800-a-month apartment. They agree between themselves to each pay half the rent. After three months, Ted moves out without notifying Karen or the owner, Laura. As one of two co-tenants, Karen is still legally obligated to pay all the rent--although she might be able to recover Ted's share by suing him in small claims court.
Co-tenants also are legally responsible for the other terms in the agreement. For example, if one co-tenant damages the property, moves in an extra roommate or brings in a pit bull contrary to the lease or rental agreement, the landlord may hold all co-tenants responsible. It doesn't matter that the other co-tenants objected or weren't consulted by the prime offender.