The short answer is that the order does not matter. You will eventually need to study BOTH areas: behavioral psychology as well as criminal justice. The order of study is less important than obtaining the overall knowledge base over time. You should try to obtain some internship experiences in the field where you can learn more about how the interaction of the law and psychology occur. For example, the areas of insanity defense and competency to stand trial for certain crimes is one place to look for some experiences. You will have to deal with lawyers and psychologists.