Qualified element names?

I have a schema that I am trying to convert to a TSD. The schema defines multiple namespaces and the elements use qualified names to choose the namespace. If I try to use a QName in an element, Tamino says is must be an NCName.

Is there any workaround for this constraint or must all element names be in the same namespace?

Yes, you need to define a schema per targetNamespace and connect them by import statements.
Thus every element defined as top level element in an schema resides automatically in the targetNamespace and need not to be qualified.