No, you don't HAVE TO, but if you want to both be on the deed, etc, then you might want to. Basically, the mortgage maker (bank) will take the better of your scores and use that to enter into the computer to spit out a loan program they can offer you. Also, they will combine your incomes to calculate the amount they can lend you - so no, you don't have to, but it won't hurt you just because your score is bad; it will possibly help you because it will make your profile a bit more robust since your income will be collectively higher.