Couple more things, is it common to do 2 majors, or do most people only do 1 major, and would it be overly difficult to major in both astrophysics and atmospheric science?
It depends on the person. I, for an example, chose to do an extended major + 2 minors. My friends often did double majors, sometimes in two very different areas. Up to you, although the most common I see are: major + minor + electives/ random units, double major or extended major + minor. IIRC, the minimum is a major and a minor or double major (no minors).
As for the latter part of your question, I didn't do astro or atmosphere science, so I'll leave this for someone else to answer.
I have also been considering whether or not to do biology (bio1011) but have not done any biology in VCE. Would it be difficult to do well in bio1011 without any prior experience doing biology?
If you like biology or want to try BIO1011, do it.
I didn't do VCE Bio, in fact, I hadn't even done Bio since Year 9 and walked into BIO1011 with just my VCE Chem and previous Psych knowledge. Fast forward three years and now I've not only studied an extended major in biology, but also done my Honours in biology (specifically, I did genetics).