I'm in a similar situation as you, so I did some research and I'm pretty sure applying for an irregular offer is the way to go if you are definitely sure you want your second preference over the first. I think you have to wait until you get your first offer, and then fill in a form for the course you actually want (i.e. your second preference), then the people in charge of overseeing admissions to that course decide whether or not they want to/can take you. I think it is advised that you accept your first round offer either way though in case you don't get the irregular offer, because if you don't you may end up with no offers :/
I'm pretty sure this is what happens, can anyone else can offer some clarification? Maybe have a look at what VTAC says about irregular offers.