Hey!
Unfortunately I can only offer advice based on last year's study design (i have no idea if things are different now - so I'd strongly suggest asking your teachers). But for the old study design: 1/2 bio only slightly overlapped with 3/4 - and the bits that did we mostly covered either in y10 science - or we re-did them in 3/4.
Personally, I didn't do the 1/2 at all. But I did put a lot of work in during the holidays - trying to get a head start. (We used the edrolo textbook too - which made the teacher mostly irrelevant).
Somewhat stupidly - I let dad talk me into reading a chapter or two of his old uni textbook - which was well and truely overkill - but did overlap with Chemistry and physics a bit.
If u do end up doing bio: strongly suggest listening to Andrew Douch's podcast ("Douchy's biology").
Anyway, Good luck with whatever you decide!