It was recently announced that internationals students could not longer take concurrent courses - meaning 2 courses (which have a CoE) at the at the same time.
The purpose of concurrent study is to give International Students the opportunity to learn additional skills for their career.
Unfortunately, there had been a large number of students abusing the system. They were coming in to Australia through expensive university courses, then quickly enrolling in lower level, cheaper courses.
The rules have not changed, but are now being enforced:
you must complete 6 months of your principle course before being allowed to apply for a concurrent course (unless you are education provider gives you permission)
If you drop to a lower level of study, you must apply for a new visa
Remember, if you drop to a lower level and remain on the same visa, you might think you have gotten away with it, but if you try to apply for any further further visa ( i.e. the graduate visa), the case officer will see that you've been violating your visa for the duration of your studies. Thus, be prepared for a refusal.