I read on another board that the lower of each person's middle score is used. For example, if husband's scores are 670, 680, 690 and wife's scores are 700, 710, 720, then the husband's 680 score is used.
It's not just your credit score that is a factor, they will also look into things such as your actual credit history, employment history and loan to value ratio. Lending standards are becoming more strict.