Two EOModels in one project

I'm adding to an existing framework with an EOModel.  Call this model A.  I'm creating some new entities which we'd like to keep in a separate EOModel.  Call this model B.  Some model B entities need to have relationships to entities in model A.  According to the EOModeler help docs, this is as simple as adding model B to the framework project, and choosing between the two models in the relationship inspector.  However, when I create a new relationship in model B, the Model popup list shows only model B.  How do I get it to also offer model A?

Denise