It ultimately depends on your career goals and the job opportunities in your chosen field. A double degree should give you enough diversity to practice in 2 different fields but also should be able to be combined into a major strength. So a Commerce/Law double may open up more taxation or corporate law positions or you could work as a criminal lawyer or accountant as if you had the single degrees. Commerce/Engineering will allow you to pursue separate financial or engineering careers or combine the two to pursue fields like project management for example. I know people at my uni (Adelaide) who do Science/Mining Engineering doubles who are majoring in Geology so they can work as either geologists or mining engineers separately but the fields obviously tie in nicely so they can create a major strength. I know I've just provided only a few specific examples but I hope you get my drift about double degrees. Employers often appreciate double degrees because it shows dedication to study and diversity of knowledge. My brother does law and most people in his law classes are doing double degrees (with engineering, commerce, science, economics, arts, health sciences, design studies etc) and they were told by a law firm that they value double degree graduates because a second degree often brings an additional set of skills to those developed during the law degree.
Word of warning though, many double degree plans reduce your number of electives in order to complete both degrees in a short period of time. So you may look at a single degree plan and see elective 1, elective 2 etc and then look at the double degree plan and see these electives have been consumed by subjects from your other degree. This isn't really a disadvantage because if you want to complete additional courses, you can so you don't have to lose those electives but it will take a longer time. Plus if you pick a broader degree such as science or arts to be your second degree, you may find the plan is for a specific major (which you may or may not be interested in) but you don't have to do that major, that is just the major which will allow you to complete both degrees in the minimum amount of time and what the uni's will generally timetable classes and exams for to avoid clashes but this can be rectified, you just may have to take classes in different semesters etc.
Hope this helps, I know the question was mainly about employment prospects etc but I just thought I'd provide other information that may be useful in your decision. Just remember though, you can always drop your double. It is generally easier to drop a double than pick one up because if you have a dodgy first year (as many people adjusting to uni do), your GPA may not be enough to pick up a double.
Good Luck!
