not familiar with university and the intrinsic details of how the courses are managed but im sure you could simply drop one if need be.
how about a science/commerce degree? you could easily major in something you are interested in and have the bonus of a commerce degree which would benefit you in making more money for the market in which you are catering for. also a engineering/commerce degree which you suggested previously also carries benefits not dissimilar to a science/commerce degree.
just keep in mind that no matter what university course you undertake, chances are you will be considered wealthy and earning above the average australian salary (which is currently over $1000 a week). i suggest to see yourself as successful (because it is your life and it is paramount you don't feel a failure later in life) that you not put money as a priority but rather things such as family, flexibility, work environment, intensity, responsibilities, hours, etc.