Don't worry if it's not what you're best at - look at my scores and now I'm considering a transfer to science to go in a relatively Med/Health Science related path.
Most people would consider me more English oriented but I find writing on demand is so difficult and if somebody gave me an option I would gladly take a Biol exam over an English essay any day.
Anyway, if you like Chem but you're not sure you should really do a double degree.
Do you live close to Monash?
If you do, I would suggest Arts/Science, or if you want and can get in, Law/Science.
You could try Melbourne Science or Arts because you'll still get a breadth but for people who aren't sure, like myself, it's always better go to general in the undergrad and then specify in the postgrad.
Considering both your degrees pretty much require postgraduate study, you won't be disadvantaged by completing a general double degree first and it will give you a lot more time to really decide which option is best for you.
I wish I had done something more general before specifying - I went into Media and Comm too soon, but I'm lucky I didn't start with RMIT Journalism straight off because I would be even more stuck.
So at the moment, i would look towards Arts/Science to begin with and think about JD and MD pathways in the future - you can decide later.
That said, you still have a whole year before you have to choose preferences and you might sway one way or the other next year.
But from someone who was very much in the same position as you - choosing between Arts and Science - I can't recommend being general any more.
I hope more people can learn from my mistakes, really