Hi there

First off let me just give you some advice in hindsight; don't stress!! While I think year 11 is really important as a foundation for year 12, I really really can't emphasise enough that you shouldn't worry. It's the last year you can do subjects for fun without having to worry about everything you do at school counting towards study scores or enter scores, so don't get bogged down about 3/4 work, it's just not worth it.
Now for subject advice. I did 3/4 biol in year 11 without having done the 1/2 and I really didn't feel at a disadvantage at all. Over the holidays I recommend working through a book called jumpstart (
http://www.secondary.cengage.com.au/secondary/science/jumpstart/) it reviews units 1/2 and gives you a little head start on unit 3. You might feel people know a little more than you in the first few weeks, but that eventually goes away and it ends up being a really fun course

As for psych I'm not too much help, but my friend who did both biol and psych 3/4 in year 11 without either of the 1/2s said they actually complemented each other quite nicely. She also did quite well so it's definitely not impossible to do both if you want.
Plus like Scooby said above me, it's also very worth doing both next year as after that they won't have midyear exams, IMO making them quite a bit more difficult.
Best of luck
