I vote yes, but there needs to be a mass transit/rapid connection between the two.
At $5.70 for one stop on the train between terminals now i shudder to thank what the charges between airports would be.
I think they better option is a high speed rail service down the east coast. That option would free up a heap of slots for international aircraft.
Id take a train from mel to syd if the 3 hours service touted started up. If QF were smart that get the Govt to build the rail then lease it off them and share the cost with a major transport company like Linfox.