Any element can be a document element.
All namespaces do is provide the illusion that you have some control over the problem space. But the fact that you must rewrite both declarations and instances in order to have something validatible suggests that there is no more control than there was before. A single document that uses element types from different sources 2. The result will be single document with no name clashes and the mapping of element instances to architectural element types preserved. The result can still be validated against any of the governing architectures.