  Coupled transitions between distinct ordered phases are important aspects
behind the rich phase complexity of correlated oxides that hinders our
understanding of the underlying phenomena. For this reason, fundamental control
over complex transitions has become a leading motivation of the designer
approach to materials. We have devised a series of new superlattices by
combining a Mott insulator and a correlated metal to form ultra-short period
superlattices, which allow one to disentangle the simultaneous orderings in
$RE$NiO$_3$. Tailoring an incommensurate heterostructure period relative to the
bulk charge ordering pattern suppresses the charge order transition while
preserving metal-insulator and antiferromagnetic transitions. Such selective
decoupling of the entangled phases resolves the long-standing puzzle about the
driving force behind the metal-insulator transition and points to the site
selective Mott transition as the operative mechanism. This designer approach
emphasizes the potential of heterointerfaces for selective control of
simultaneous transitions in complex materials with entwined broken symmetries.
