It's a very compatible degree. Heck you don't have to do 'compatible' majors, some people are doing 'interesting' combinations. You can do totally different things in each degree if you want!
Based on my experience with fellow Comm/Sci People, the most popular majors that people pick in this program are accounting/biotech, finance/biotech, maths/finance, maths/actuarial and something-commerce/psych. I don't see many people doing Geology/Accounting unfortunately, but hey, you could be the first!
Yes, in Commerce/Science you can pick up a career from either field, or combine them. You'll find that people who think Commerce is all about $$$, and Science is all about Research are the ones that cannot fathom how such a combination came about. In reality, science is driven by $$$, and there is a science behind commerce. OK, I'm twisting things a little here, but you get my point -they are not two mutually exclusive fields.
If you are really unsure, here is some practical advice, pick up Commerce/Science. You can always drop back to Science or plain old Commerce with a simple form, but going the other way requires you to go back through UAC etc.