From what I heard I was told that doing a double degree doesn't always get you the single job
e.g. Doing Law and Computer science will land you as working as a lawyer specialising in cyberlaw which I don't think is always the case
Say you did Law and Psychology, there would be a range of careers available to you. Keep in mind that you don't need to work as a lawyer, you can work as a psychologist (given that you have the psychology degree)
That being said if you do something like psychology/law at uni you could (if you wanted to use both degrees) become a forensic psychologist, a behavioral scientist or (obviously) a lawyer where your knowledge of psychology will help you better understand and relate to your clients which can aid in getting your questions answered honestly and in providing good legal counsel. Coursework in psychology and behavioral sciences can also improve your negotiation, resolution and problem-solving skills.