I'm a Year 11 studying both Psych and Bio accelerated.
I joined the U2 Bio class mid-way last year because it took me 6 months to convince the faculty to let me accelerate two subjects. Better late than never, I suppose. Bio is far harder than psych in terms of raw content to remember, and I am doing significantly worse in Bio compared to psych last year, and especially this year.
Psych is quite an easy and fun subject compared to Bio for me, since the content although still possibly reasonably heavy, it makes far more logical sense in my head to memorise and I'm personally invested in the field of psychology for personal reasons, which helps with motivation.
If you want advice, take these points:
a) It is doable. Accelerating Psych and Bio works fine. Some content you will learn in Psych actually translates to Bio and vice versa (e.g. neurons, neurotransmitters, Na+ and K+ ion channels)
b) If you're like me, you'll find psych easier than Bio and Bio may or may not kick you up the backside for how unintuitive it can be to revise compared to psychology.
Your experience may vary, my experience could merely just be due to other circumstances around myself and my school's teachers, or it could be applicable as well. Just be careful with what you pick.