RDA has a category of relationship designators, applicable to Works and Expressions, called “complemented-by”, which is a subcategory of accompanying relationships.  In a complemented-by relationship, neither of the two Works or Expressions is considered predominant.  An example is a libretto and a musical work such as an opera.  Complemented-by relationships differ conceptually from other accompanying-work relationships, where one Work or Expression is predominant, e.g. a work and its appendix, addenda, errata or index.

In BIBFRAME, the definition of the property bf:accompanies is “adds to or issued with”.  This seems to embody the narrower approach of one predominant entity.  The definition should be expanded, or a new subproperty introduced for complemented-by relationships.