Hello hellomeyou,
I am a JC1 student who has just started my new school life for 3
weeks and I can tell you some stuff regarding the sciences.
Firstly, I am sad to say that you cannot take H2 bio because the
pre-requisite states that you need to have O lvl knowledge, whether
as a pure or combined subject. In fact, H2 bio, chem and physics
all require O lvl knowledge, which means you can only settle for
chem and physics.
Secondly, you must ask yourself whether you wish to have a
science career (engineering,chemistry,medicine,dentistry) in the
future. If you do, then you MUST take H2 chem and H2 physics. True
that you will be slightly disadvantaged since you are from combined
science, but it doesn't matter because if you have the passion and
drive, nothing can stop you. However, you must check which JCs
allow combined science students to take 2 H2 sciences. Just to add
on, H1 science is useless for admission into a science course in
uni.
Lastly, all the best for your O lvls!!! Do not be overly worried
about this now since you still got 2 months to think about it. But
nonetheless, it's good that you are thinking ahead :D