I have never been to Melbourne Zoo, however, can definately comment on the enclosures at SWW...
They are amazing...
The design is fantastic, and frankly, they mixed species/multiple animals makes for a MUCH more interesting display...
I believe the key, as has been addressed by a few others, is that SWW provide exceptionally large and well-designed display enclosures to accomodate this. In fact, the first time I attended I could not believe the height and size that was available to the Blueys/Shingles - I attended again recently and it now makes a lot more sense, they have added Gouldain Finches to the same display...
I recently watched a keeper feeding the Frillies and Hosmers (If I recall) at the same time in the same enclosure, the difference became clear - the Frillies rarely leave their trees and when they do it is to get down to the ground to feed...the Hosmers dont really leave the rock ledges...it works...and looks great! (In fact, the same enclosure also used to house Tristis as well)...
I would still hesitate to advice too many keepers to travel this route as I think, in general, enclosure size in most private collections makes this more difficult - in saying that - if you provide an adequate enclosure I dont think you could put together a much more impressive enclosure than a functional mixed species setup...