Okay, I'll help you out with the engineering stuff cause I'm from Adelaide and hence, don't know much about actuarial cause we don't have a degree here but basic engineering information is universal across major Aussie unis.
An engineering single degree is usually 20-25 hours per week minimum, depending on how many labs you have on that week. Most labs are about 3 hours but I am about to experience the "joy" of 7 hours labs this year. However, labs are generally not every week and the *general* trend is that your contact hours will decrease as you progress in your degree BUT more time will be dedicated to research projects and coursework i.e the time is transferred elsewhere.
Double degrees have similar hours, maybe a few more because some of the subjects in an engineering single degree plan are replaced with subjects from the other degree. You will generally be at uni 4 to 5 days per week because engineering faculties generally present fixed timetables i.e only one lecture/mass tutorial time etc and they like to space it out.
Personally, I would do engineering followed by MBA. An MBA will hold more ground than a B.Commerce and I believe experience in the engineering industry will help you make a more informed choice about what sort of path to take within a MBA. You don't want to do a B.Commerce and think a certain path will complement engineering and waste time and money to find it is a useless piece of paper cause you picked the wrong major based on someone else's poor advice. YOU need to see what will complement YOUR engineering career because many engineering careers turn out very different to what is first anticipated. You will also find that engineering also teaches basic financial knowledge. I started in a Law/Commerce degree and transferred to Engineering after a semester and found that components of my Intro to Engineering and Planning and Design courses covered the Time Value of Money component of Accounting for Decision Makers and the entire Principles of Macroeconomics courses I had completed for my B.Commerce.
I don't think engineering double helps too much unless it is something like Petroleum Engineering which has Chemical (petrochemical, fluid mech), Mechanical (Drilling Engineering) and Civil (Oil Rig Structure) components because you can tailor your career path based on those doubles. Something like Civil/Mechanical isn't very useful and I don't think it can be done unless you want to spend many years at uni. This is because although there is some overlap in the first 2 years (statics, dynamics, computing and maths are the main ones), you have to learn different things. Civil focuses on soils, water, structural materials, environment, Mechanical will cover dynamical control, thermofluids, engineering design etc....I know this is very vague but I hope you get what I mean. Double degrees only work if the degrees have interchangable subjects or many common subjects.
Hope this is of some help.
BTW: The doubles and courses I have listed are for Adelaide Uni so you will have to do more research yourself but I'm just trying to give you a general idea
