You can require a co-signer if their credit is less than stellar, if they have no rental history, if their employment history is sketchy, etc. Essentially, it is your right to require a co-signer if they don't meet your criteria for renters. If you do accept a co-signer, have them apply just like any other tenant and screen them well. Then have them sign a co-signer's agreement stating they will be responisble for any charges that the couple don't pay. You can even require two if you wish. (Might not be a bad idea, if they're not married. This way you can go after someone from each side for unpaid debts.) You want to be sure that any co-signer is collectible, usually by owning real estate.