The XML spec. puts no constraints on applications, so you can do
whatever you want. It does not define "validation" for hyperdocuments
constructed by gluing together bits through XLL. If you wish to invent a
definition of validation that allows this, for the purposes of your
application, then you may do so. An easy way to do that would be to read
the two entities, and pipe it through a processor as a single XML
document. But no, it is not the processor's role to do this for you,
because the processor need only support the concept of validation
described in the XML spec., which has nothing to do with XLL at all.
Paul Prescod