First of all, I'd immediately say that it isn't worth it doing both, if the circumstances were that you would do all of them at school. This is because some universities do not accept both (geography and geology) and would thus recognise only one, not to mention that you don't need geology to apply to any university AT ALL (I believe) - I'm in year 13, doing mathematics, chemistry, physics and economics and applying to do earth sciences/geophysics, and haven't found any courses that required it.
However, if you believe you can handle all of the subjects using efficient and effective time management, and do well enough to get the grades you need, there is no doubt that it will provide you with a legitimate example of you enjoying the subject and seeking understanding.
In conclusion, I'd recommend NOT doing five AS levels, but if it is something you feel strongly about, there is no doubt that it's possible.