I studied those two sections at once. Try studying for AUD for an hour, then BEC for an hour, then AUD for an hour and so on. It worked for me because it's slightly less boring that way, and it's a good way to make sure you're making progress with both sections. Have you already gone through all the multiple choice questions? If you haven't I strongly recommend you go through them all just for the sole purpose of exposing yourself to all the topics. Unlike FAR and REG, AUD and BEC involve a lot of common sense and just logic. So just scheming through all questions will knock out up to 50% of all questions, and then you can just focus on the other 50%.