I studied Chemical Eng. a couple of years back. There were a number of commerce doubles in my cohort and I can tell you from experience that the majority of them were lesser engineers. There were exceptions of course, but alot of them either didn't have the talent to focus in two areas or were simply wasting their time with engineering and diverting their energies from commerce where they belonged.
Let me tell you something about an engineering degree. It's diverse, especially in my field, and there is no way in hell the core subjects are going to make you a competent practising engineer. You can't do half the work and expect to come out with the same skills as someone who has covered a good range of electives. I can tell you from experience that the electives are what shape your interests and help you stand out from a commerce double who can barely do the basics of the course. In my case in addition to normal chem eng core units I studied mining, polymers, green engineering and waste water treatment to list a few. These all helped my job search and my continuing professional activity. The basics are just that - a starting point for further engineering study.
I'm sure that the same will be true of commerce studies, although I cannot speak from experience. Also with regards to employment I can't say how it affects your chances other than to say now where I looked at had any commerce, arts, law component to the application so I doubt it would have helped. There are positions for double degree engineers, an acquaintance is a lawyer/engineer, but these positions are rarer than straight engineering. Whether you wish to hybridise in the workplace as well as uni is another question for you as well.