I studied Engineering/Business at UTS and knew many others studying Eng/Business or IT/Business, most people will generally get a career in one field or the other (for Eng students it's generally engineering, for it/business it's usually business from what i've seen). I'll answer about Eng/Business, but it should apply to Science/Business too.
I would say the entry level career prospects aren't all that different to the single Engineering degree, any position that might require some business knowledge would likely get taught on the job. When it comes to managerial positions/career progression, an MBA later is probably the better choice for networking and symbolic reasons as well as being tuned to that kind of position (as opposed to undergraduate business that might assume you want to become an accountant) so I wouldn't take undergraduate business just for that.
But having the knowledge/degree might still be useful and an extra year of study is not so bad - I'm definitely glad I studied business and use what I learnt very often. Another advantage is the workload of business is much lower than engineering and because business has such high enrolment numbers, the classes just feel a bit more polished and well run. Additionally, every single business subject I studied at UTS could be taken entirely in the evening on at least one semester each year which was great for balancing work and study.
To better answer about career prospects and what unis are good, we'd need to know what kind of science you want to study. UTS have some nice and new facilities for science and i'm sure there's probably some areas where it's the best. But generally speaking, UNSW and Usyd get more research funding/have better research connections so that's what people will likely say. But from your ATAR goals, I suspect you would be in a position where you can get into Sci/Business at UTS but only straight Science at UNSW or Usyd and would have to try internally transferring if you still wanted to study business.
