Scoring a 20/20 in any subject is easy to say, hard to execute. Doing 2 full subjects in year 11 can potentially do more harm than good. Even though you will dedicate all your time to these subjects, you may not have developed the crucial problem solving skills required in mathematics. This comes from experience and time. Also will be taking time away from developing solid foundations in yr 11 subjects such as chemistry which is essential to ace year 12 - don't neglect essential year 11 subjects which will be assumed knowledge for year 12. I have seen this be a downfall for many students. End of the day, it is your choice and confidence in your own capabilities. Theoretically, the more year 12 subjects you do the better variation and chance you have at forming a higher ATAR but then again, theres only 24hrs in a day and only so much you can cope with. I would recommend doing biology with either of English/RP which we are hoping not to count in the final ATAR. Also for note, never heard of this human bio subject - is it different from regular biology and is it new ?