I asked the deans this question yesterday/today so to clarify:
At USYD it used to be compulsory to do a double major in commerce even combined with law. This year it is more flexible, and although no longer compulsory, it is most common.
At UNSW, the commerce faculty don't really accommodate a double major within the combined law structure. Some people do choose to do the overload of work, and although the faculty does work with them on that, they're having difficulty properly recognising the double major. He did however add that they are looking seriously into that this year to see how things can change...but he added that it really is too much work and cannot see it really taking off.