Can one dwelling be occupied while the other is still under construction?

Yes, potentially. One dwelling may be occupied while the other is still under construction if the completed dwelling is covered by an appropriate Certificate of Occupancy and Use.

The Building Act 2004 allows a certificate to be issued for a completed part of a building where that part satisfies the applicable requirements. The remaining uncertified part cannot be occupied.

This means it is not automatic that one dwelling can be occupied simply because it is finished—the required occupancy certification must first be obtained.

Reference: Building Act 2004 (2004-11) – Sections 69(3), Certificates of occupancy & 76, Occupation and use of buildings - https://www.legislation.act.gov.au/View/a/2004-11/current/html/2004-11.html