As your sibling comment says, there is only so much market for luxury housing. If the land, permitting, etc. exists to build two houses, then two will get built. And if there's only sufficient demand for one of them to be high-end, then only one will be. (In aggregate on larger scales of course, not literally any given two houses.) It's not necessary to force a given developer to build two houses instead of one; if the economics make sense, and are predictable, the developers will come.