Unfortunately only common sense is the answer. For example, overflying a non-towered aerodrome at 1500ft en-route I will definately give a broadcast and listen to the CTAF. If I'm at 5500ft AGL I'm unlikely to give a broadcast, but will listen to the CTAF for situational awareness about inbound/outbound traffic.
Your example is a little different. If I had only one comm I'd listen to the CTAF I was closest to, or had traffic I was more likely to come into conflict with. If you have two comms, why not one CTAF on both? The only problem then is whether you'd want to be listening to Centre at the same time.
I haven't been past Tyabb or Tooradin for a while so I can't give specific advice.