There isn't such a thing as a joint will; you have to have separate wills. You can have a mirror-image will though.
QUESTION: Listener asks if she and her husband should have their separate wills.
ANSWER: You must have separate wills; you can’t have joint wills. You are not Siamese twins. You have can mirror-image wills, where everything is alike except the names at the top. He gets everything if you die and vice versa. If you both die, your kids get everything.