Xerces-C++ for Validating Against Multiple Schemas
Xerces-C++ is Apache's C++ implementation of the Xerces XML parser. It turns out that it ships with a simple example program, stdinparse, that can validate your XML (which many tools do) against multiple schemas simultaneously (which few Open Source tools do).
A sample command line could be:
$ ./stdinparse -n -s < /tmp/10.5072__FK250925-xerces.xml