Of course you can.
But the issue that arises from your question is that, why on earth is the half-yearly weighting so ridiculously high? It's understandable if it's Mathematics, but if it's from a different faculty.. It's kind of unfair because the weighting is so high, and it's ideally, only testing you on half the content (as opposed to the yearly which is testing you on the entire course work).
But yeah, just try to stay within the ranges of 1-3 in the rankings. Have your teachers explained how the HSC marking works? (How they determine your mark in particular)