When examining table relations in a database, DevForce can’t tell the difference between a 1-to-Many relation and a 1-to-1 relation, so it generates relations as 1-to-Many.  You can override this assigned cardinality in the Object Mapper to make the relation 1-to-1; doing so will change the nature of the relation properties that are generated into the two entity classes.  Instead of a relation property representing a collection, it will represent a single related object.

You might want to look at the Instructional Unit on Generalization Hierarchies.  There you will find a discussion (in the PowerPoint slides) and examples of different ways to approach your development in the  case where two business objects have this tight coupling.