Moving Foreign Keys

We have a large Physical Data Model (PDM) that we are splitting apart into smaller (subject-area-focused) PDMs. We can move tables from the larger model to the smaller models but cannot move FK relationships to the smaller models along with the tables. How can we move both the tables and relationships from model to model?

Additionally, once we have the larger model split apart, we need to maintain FK relationships that cross subject areas. We can create shortcuts in our smaller models that reference tables in other subject area models but need to ensure that FK relationships between the actual tables are maintained and want to understand the best way to do so.

Lastly, are we approaching this properly? Should we be splitting the model into multiple PDMs, using packages, some other structure? What is best practice?