UNSW > USYD for Engnineering. I'm not sure if the project management side of the degree is tailored to engineering, but if it isnt (i'd presume so) it would be no different to doing combined eng and commerce/business - unless you choose to major in something random for the commerce side ofcourse. You wouldn't want to get a seperate degree in commerce or project management etc. a lot of engineers who are on track to getting management positions end up doing MBAs and from what i understand your employer often pays or subsidises that. That is something to do after you have an absolute minimum of 5 years experience. I can personally see myself eventually getting an MBA anyway.
Just having a project management, commerce or business degree isn't going to magically open up your job prospects but it could give you an edge over competitors for some jobs. Either way it's just an extra 1 year on top of the normal degree, gives you bit of variety in subjects and pads up your gpa since commerce subjects are easier than engineering.