Again, from the project I'm currently working on, there are times when you need to have multiple attributes as the identifier key for a entity/table (I'm currently using a unique index in the multiple attributes to bypass this feature).
At this point the platform doesn't allow that, when you right click->Set as Identifier a attribute it changes the key to that attribute instead of adding to the key.
The idea was to ask the developer if he wanted to add the new attribute to the key or change the key to this attribute. Should ask this everytime just in case you mess up and need to restart this process
I'm guessing the problem here would be when you create a foreign key to a entity with multiple attributes as the key, you need to add the entire key and not only one field, so you probably can't do this automaticaly