You can put down both, it doesn't really impact the chance of an interview at JCU Med/Offer at JCU Dent. Given that you are not applying to other schools, you can use 3 of your six slots for JCU if you choose. (Those being Cairns/Townsville for Med and then one campus for Dent, I think it's Cairns)
Put both down and fill out the JCU application online. Whichever you want more, put first and if you don't get a second round interview for JCU Med put JCU Dent as your first preference and wait for January Rounds. If you don't get a JCU Dent offer, then put Bond as your first preference if you are able to afford!
Make sure a predicted atar is sent to JCU for first round interviews, otherwise you'll be considered for second round, which has a smaller number of interviews. Keep in mind that JCU Med is only allocating interviews based on where you went to school, so we have no idea how it will work. Write a killer JCU Dent application! (I've seen low atars get offers)
Work on your atar! A predicted is never a proper indication so make sure it doesn't get to you too much, you never know what will happen!
Good luck!