Psychology fits well with Biology as the biological explanation is one that you have to do for almost anything. You might find that easier when you're studying more advanced Biology as well. Also, the stress topic is particularly biological as you need to know pathways involved, effects on the body etc. It was one of the hardest topics for me and I definitely think it'll come more naturally if you're doing Biology as well. There's also some Maths involved and you might find Research Methods easier in general when studying Maths anyway.