There isn't much stopping you, provided there are no exam clashes and you can organise the practical component. However, doing 5 is a lot of work and Chemistry and Biology are quite difficult to self-teach, as so much of the exam is, unfortunately, familiarity with how the examiners want you to answer and it happened to me when I did Chemistry (I didn't do Biology) that I would lose marks for missing out something specific from the mark scheme even though my answer was otherwise correct and sufficient.