From what I understand. It isn't possible.
double degree means 1 major from each degree.
i.e. commerce (major in whatever you money-making freaks do) + science (i.e. medical microbiology)
Trust me, once you hit the end of 1st or 2nd year, even just completing the science degree part kills you.
Had a buddy who overloaded almost every year, he's now finishing half a year or a year earlier. B Com/Science.
You'll find most ppl who do double degrees are shit in one side. i.e. most commerce people want money, but can't think in a scientific way apart from supply and demand. I knew the smartest guy, he'd do heaps well in commerce but get ok marks in science. If it weren't for us passing him our course books + exam tips, he'd be in the pits.