The two degrees you do will be completely separate, so your non-law degree doesn't influence your law degree in any way. A law degree alone is sufficient to work in pretty much any area of law. So to answer your question there isn't any substantial disadvantage in having an arts/law degree if you want to work in commercial law.
Keep in mind that there are law electives that can gear you towards certain areas of law such as commercial law, human rights law, international law etc.. I recommend you use your non-law degree to do something that interests you.
There is also not much difference in terms of employability between a single law degree and a law double degree. The main difference between the two is the workload you will have at uni. In a double degree, your workload is split between your two degrees, which might lessen your workload than if you were just doing straight law (although it will take you longer to graduate). You might also get more electives in a straight law degree but you'll have to check on that.