Since they established it between themselves, it doesn't matter who didn't hand you their portion as far being up to date on rent payment. As a landlord and more importantly as the legally binding contract BOTH tenants signed, rent is $1800 period and BOTH are responsible for that payment regardless of there personal arrangement between themselves. If you financed a car with another person, and the payments were not paid in full each month, the bank is not gonna care who didn't put in their share. They are not gonna say since you paid your half you get to keep and use the car but the other person is banned. They will go after both of you and repo the car.