I did both at AS, my exam boards were OCR for Law and WJEC for Psychology but both involved a lot of content to memorise, Law especially. However, if you genuinely enjoy the subjects you should be fine. I'm taking A2 Psychology now and enjoy it, some stuff is boring like relationship formation but I'm finding the forensic aspect of psychology really interesting.
It really depends what you want to do after a-levels though. Some top unis will prefer facilitating subjects, so you may find it harder to get a place if you carry both Law and Psychology to A2. So if you pick either Law or Psychology and have two facilitating subjects at A2, it's easier to keep your options open.