Semantic equivalence of xml documents

Hi

We have defined a pretty large xml schema document. Based on this
schema, our customers build instance documents. What we would like to do
now is to compare two of them at a time to their semantic equality. Is
there any tool which can handle such requirements? As the input it
should take the schema docs and two instance documents. The only url I
found to this topic is
http://www-128.ibm.com/developerworks/library/x-diff/index.html . What
we also tried is XmlUnit but our documents seem to be too complex/nested
for the tool.

Any help is appreciated.

Thanks
Yves