Hey! Welcome to AN.
I didn't do two year 12 subjects in year 11, however I did one in year 10 and one in year 11, and have done both bio and further.
The first thing to consider is would your school allow you to do only 4 subjects in year 12? I've heard of some schools that force students to do at least 5, even if they've already done 2. If your school is one of those then your workload in year 12 won't really be changing.
From the way your post is written I'm assuming that you're not currently doing 1/2 and are planning on going straight into 3/4 next year?
If your school would allow it, then joining a 1/2 class for second semester would probably help you for doing the 3/4 next year.
The other thing to consider is how well you can manage your time at the moment. For me, I barely spent any time on further outside of class, except for the occasional practice sac (keeping in mind that I had done 1/2 and there is a fair amount of repetition between the two). However, at the end of the year I spent a significant amount of time doing past exams. Whereas for biology I regularly spent time on it outside of class throughout the entire year.
If you think you are or can be organised enough to keep up with the work, and not neglect your other 1/2s then I'd say go for it. Only doing 4 year 12 subjects in year 12 certainly does make the year a bit less stressful.