In my opinion, (well this is what I did for bio)
I wouldn't recommend spending 75% of your time on PE - although all you need is an 'S' for your 1/2s, spending only 25% of your time on them would mean you'd have a lot of catch-up work to do over the summer holidays before year 12. Definitely spend more time on your 3/4, but don't neglect your 1/2s as the knowledge + skills you build in 1/2 set the foundations for your 3/4 subjects in year 12.
Something I found really useful was to be as efficient as possible in my 1/2 classes - I would get as much homework done in class (e.g. textbook exercises, worksheets, essays) so I could spend more time revising/studying ahead for my 3/4 at home. Also, make sure you know what areas of the course in your 1/2 subjects are useless for 3/4 and don't waste too much time studying for these. You can find out by comparing the 1/2 and 3/4 vcaa study designs or by asking Year 12s.
About 2 weeks before each bio SAC, I'd spend as much time as I possibly could solely on that. As soon as term 3 was over, I pretty much let go of all my 1/2s (aside from preparing for a few tests), and focused entirely on my 3/4 until I'd done the exam. I was quite lucky that all my 1/2 exams were a couple weeks after my 3/4 one - if your 1/2 exams are before you might have to start exam prep a lot earlier to stay on top of those.
Hope this helps