A related comment, probably not relevant to your particular requirements, but I hope not off-topic:
I have often found that when I thought I needed to put multiple xml files under the same document element, very often I got a better solution by saying "our exchange format is not XML it is XML-in-ZIP" and keeping the documents separate. And building infrastructure and services accordingly.
(In the publishing world, at least, you never know when there may be some attachment or graphic or inconveniently-large document lurking. Ready to spoil the nice "only XML" architecture. )
Cheers
Rick