Depends what you want to do with your life though... doing a double degree for the sake of doing a double degree (surprisingly this happens more often than you'd expect) just leads to more HECS debt and a longer time before you start actually working. Think about where you might want to work in the future and what kinds of roles you might be interested in, and then evaluate whether doing the double degree would benefit that.
I work in the Commerce field and have found people with Engineering or Science backgrounds to be very strong at certain aspects which are typically glossed over by the typical Commerce graduate, but it needs to be something you are naturally inclined towards and interested in.