hey! i did two 3&4's last year, so maybe my experience can help

taking more than one is definitely beneficial, as it decreases the stress and pressure in yr 12. if you don't perform as well as you expected this year, you always have another opportunity next year, or if you choose to take <5 subjects in yr 12 you'll have ALOT more time to focus on them and get a high study score!
although my school absolutely hated me doing 2 subjects, i learned so much in terms of exam preparation/organisation skills/study habits. it IS a little stressful at times (depending on what subjects you take, of course)! although it's important to note bio is a content-heavy subject that does require a considerable amount of time for consolidating concepts/practice exams. from what i remember, some of what's taught in 3&4 doesnt need unit 1&2 (eg, evolution), but i'd recommend looking at stuff like cell replication as that may be tricky to understand without unit 1&2 knowledge.
something i faced difficulties with last year was balancing between my year 12 and 11 subjects- i'd say prioritise your 3&4's in terms of being ahead (beneficial but you dont have to!), prac exams, preparing for SACs, etc, but make sure to complete homework & other tasks in year 11 subjects, as what you learn in units 1&2 sets the foundations for 3&4.
ps: i noted that you're studying methods/specialist 3&4 to prepare for next year, which is great! but you'll have lots of time next year to do practice exams for them, so rushing through the whole course now isn't necessary. of course that's just my opinion, but spending as much spare time on your 3&4's this year will assist in ensuring that you achieve a high study score!