Shared houses are an edge case, yes if you don't want to share your private data (income, health, etc.) with other members of the share you can do your own census form but for the vast majority of Australia all members of a household go onto a single census entry. In your example you'd only be putting in an entry for the share house you were staying in on the night of the 9th, it doesn't matter one bit that you live in two share houses - the census is just a snapshot of August the 9th.