SQL Developer

I have a logical model in SQL Developer Data Modeler. Entity A has two 'incoming' foreign key relations to entities B and C. Both releations have the same optionality and cardinality. I want to place both relations in an arc. I see the buttons in my toolbar menu, but they are not enabled. Even when I select both relations it is not.

Bringing the logical model to the relational model creates foreign keys. These foreign keys can be selected and when I do that the 'Create new arc' button in the relational model toolbar is enabled.

What am I missing in the logical model ...? The documentation doesn't give a clue.

Erik