There are plenty of different job positions in the banking/finance sector. If you are looking at some of the more high paying/prestigious offerings such as Investment Banking or Corporate FInance/Consulting - then a double degree in Commerce/Law will probably aid you much more than an honours in a commerce major: PROVIDED that you maintain a high average in both. This is because most IBs look for graduates who have completed a very comprehensive and rigorous degree (law is one, engineering being another) - these degrees simply provide evidence to IBs that the applicant can handle the enormous workload required. In fact, the concepts needed in IB/investment consulting is fairly rudimentary (most finance graduates/maths or statistician graduates would have no problems understanding the basic concepts needed to do the job) - it is simply the hours worked (8 in the morning to well over 10 at night, 5-6 days a week during project submissions and presentations) that would drain even the most dedicated employee. Therefore, IBs don't want to take on graduates who will simply quit after a short period with them - and the best way to thin the herd is to select the most motivated and hard working graduates out there (and the only quantitative way to do that is the ones who achieve the best marks).
Doing the above is also only one of several criterias that these firms look for in prospective candidates; the other is someone who has very good related work experience and/or outstanding extra-curricular activities. These prove to them that you will be easier to train and would work well in a group of people during large project submissions.
However, having said that - graduating with honours in Commerce will not really put you at a disadvantage either. I know plenty of people who have graduate with honours in finance/economics and even accounting who have gone to work for Credit Suisse, JP Morgan and the other big IBs straight out of Uni. Mind you, they achieved averages of 85 - 90+ during their studies and all graduated their honour program with a H2A or above (anything below that is considered a waste of doing honours IMO).