This is the part of ISO DSDL that overcomes RELAX NGs single namespace bias; actually it is a pretty useful thing all round: in fact is so powerful that the ISO DSDL working group moved away from providing a general-purpose pipelining framework, as originally envisaged, because NVDL addressed so many of the issues.
NVDL is not so complimentary to Schematron; Schematron already allows multiple namespaces fine. But NDVL will provide a great mechanism for allowing your to selectively dispatch different parts of your document to different validators. So you can pick the best schema language for the job. Or, as is more often the case, you may be working with different vocabularies each defined in a different schema language (DTD, RELAX NG, XSD, Schematron, etc).
NVDL is mainly based on James Clark and Murata Makoto’s work, but I had a little input into it with my Namespace Switchboard too. Of course, a lot of the final form of the language has to do with the SC 34 WG 1 at ISO.