My Two Cents...
2 airports in Sydney won't work. Reasons is that a lot of pax inbound have connecting flights. So a pax coming in from, say, Dubbo, needs to connect with a flight to BNE or even LAX has to travel somehow to the "big" airport.
YSSY is small in acreage compared to other airports. The conjestion isn't in the air its actually where to park all the aircraft; and jams on the taxiways.
We need to make the tough choices soon. A new airport out of town with a high speed land connection to the city. Before the urban sprawl leaves it too late.