If you're not going to enjoy commerce, don't do commerce. It's a lie that most BSc students at Sydney are trying to use it as a stepping stone. I know my brother did, but now that I do it, I don't know anyone with that plan.
Double degree is an option, if you WILL enjoy commerce.
But also remember that you can change if you don't like one or the other. It's not fixed now.
Finally, most people here probably have no idea in terms of the ideas your parents are expressing. It it true that most BSc / adv students don't know what they're doing with themselves after uni. I'm one of them. But it gives you a chance to decide.
Also, [so that wasn't really finally], Commerce isn't business. If you want to become an accountant, a stockbroker, a banker, or someone involved in finance, fine, do commerce. But if your parents want you to become a businessman, you head where you head and do an MBA =P