Doing a 1/2 subject in Year 10 can be tricky, especially considering that you're moving over a whole year of content. I wouldn't advise doing two, unless your Year 9 teachers are absolutely sure you can handle the workload.
If you do 3/4 in Year 11, and don't get a good score, you can in fact redo the subject in Year 12 as a 3/4, and your better study score will be counted towards your ATAR. I'm not sure switching to another 3/4 subject is possible without the doing 1/2 class before, although I think it depends on the school and the subject. I'd ask your Year 9 teachers for advice on what to do, and if your school has a VCE coordinator, try talking to them too