Engineering and Science are quite different fields. Science leads to more research and academia based job opportunities. Engineering is more related to the application of Science to the real world.
Now, it depends where you end up in Chemical Engineering. In Chemical Engineering you can end up working on factories or industrial sites (similar to what you would expect from HSC Chemistry course option module "Industrial Chemistry", except you would do the designing!!!) or you can end up doing Industrial Chemistry as an engineer (which is more lab design based and you will be in labs every day).
Leaving uni with a Bachelor in Science (Chemistry), you could possibly end up doing an Industrial Chemist's job too (though they do have their differences in a way so it depends on the job specification). However, if you do Chemical Engineering, you will end up with that flexibility to work on sites.
So having a BE (Chemical Engineering) / BE Advanced Science (Chemistry) would leave you more open to different types of jobs. In a sense it won't make you too much more employable as a Chemical Engineer (so a minimal effect), but it will make you way more employable as an individual altogether comparatively as it leads you up to more flexibility for different types of jobs in the end.
However, I dare say that doing Chemistry in university would help a lot as a Chemical Engineer, though it is not a requirement to be a Chemical Engineer. And I would not say the Chemistry in Chemical Engineering is basic at all. You will learn all the difficult stuff, but in engineering roles, less of the work comparatively will purely be Chemistry compared to a more Science related job.
Looking at the UNSW handbook, it seems that the only time you start doing "different" courses is in second year. I am not sure as to what degree of difference exists.
My advice is to do whatever you like that makes you feel happy. If you want, and you get the ATAR for it, do the dual program BE / BE, then drop whenever you like to (first year you will have common units)..