Well, this is a complicated undertaking. How you go about it all is not for me to say. Can only discuss the tax implications.
Most straightforward from a tax point of view is one party purchases in their name, building demolished, subdivide, then transfer new block to the other party. Then only one party has to deal with the CGT as well as state taxes such as stamp duty.
Market value of subdivided block as at transfer date less 50% purchase price = gain (or loss). That is assuming blocks are same size. (50% of all costs to subdivide included in cost base)
Important thing is to keep record of all costs, including demolition of building.