Core does not have non-constituent records in the way that Raiser's Edge tracks non-constituent relationships.  The setting to only create a new constituent for one of the pair is specific to the plugin as an attempt to work with those schools who prefer to have a shared constituent record for the married couple.  However, if there are individual constituent records for both members of the spouse relationship, then ConnectRE will try to match and link the corresponding Core records to each of them individually.  It will not revert the spouse record back to a relationship record.