Not too sure about that...
It's more of a school thing than a VCE thing.
Most schools request that you choose your subjects before the end of the school year.
I would ask what your school thinks about it.
There's quite a big difference between doing a 3/4 subject in year 12 to doing one in year 11.
In year 12, you can do almost anything you want (provided you do at least 1 english subject and you don't do specialist without doing methods), regardless of whether you've done units 1/2 or not.
In year 11, you can pretty much only do what your school lets you do. If you were talking about doing units 3/4 chemistry or specialist in year 11, then you would have to ask your school about it and the answer would probably be no (unless you are an exceptional student).
But since accounting is considered an "easy" subject, they should let you do 3/4 accounting in year 11 without having done units 1/2.
Quite a few people at my school did unit 3/4 "easy" subjects without doing units 1/2 whilst they were still in year 11, such as psychology, biology and further maths.
Most people that accelerate methods usually have to do units 1/2 though.
But in answer to your question: Yes, you can repeat a subject if you didn't score as well as you wanted to in year 11.