Does ur sixth form have the option of taking 4 A-levels ? If so, take both and once you’re a few months in, u can drop one of the subjects if u want, or even at the end of year 12. Or u can continue with both. That’s what they did at my sixth form.
Another thing is to maybe pick physics, and once u start ur alevels, stay a month or 2 in, and see whether u like it or not. U can then change to bio ( I’m saying this bc the first 1-2 months of lavel bio is the easiest thing ever if u put ur head into it, it gets much harder after that).I’m not sure about ur sixth form but u could change subjects in mine.
Another thing to consider : don’t stress urself too much imo, u can even change ur subject choices on results day ( as long as u have the grades they ask for)sooooo you’ll be fine
hopefully you’ll have a clearer idea then