I did legal studies last year, and am doing national politics this year. In terms of knowledge required, there's a few areas of the course that are duplicated between the subjects. However, the application of said knowledge is quite dramatically different -- I found that legal was quite descriptive, whilst politics requires a more analytical approach.
Having done legal studies, I think in some respects is somewhat detrimental, since I often find myself answering questions like I would in legal, which isn't exactly optimal. My legal teacher last year said that some politics kids often got carried away in their legal exams writing things which, whilst very sophisticated, weren't relevant to the legal studies course and thus didn't get full marks.
The main areas where I've found similarity thus far:
- Responsible & Representative government (weeks are spent on these principle in politics, compared to a few periods in legal)
- Electoral system (as above)
- The Australian Parliamentary System (legal studies basically describes how these work, whereas politics requires analysis on how well the institution works)
- Democratic values (extremely important in politics; there's mention of it in the legal studies course as part of the human rights section)
I'd certainly advise doing politics and legal studies, not because there's any sort of synergy between them, but because they're both really interesting subjects.