XRefs between instances using different content models

[Paul Eisenberg <paul@appliedcontentllc.com>]

Hi All.

Odd question here... Let's say I have a two sets of documents that are 
based
on different content models. One set uses a customized version of DocBook
and the other set uses the FDA's SPL (Structured Product Labeling) standard
Pharma. The custom DocBook model includes XREF and the SPL model contains
something called a "...TextFragmentReference" (the actual name is a bit 
longer
based on a naming convention).

I want to know if it should be possible to create references from 
instances of
the custom DocBook document type to targets located within instances of the
SPL document type? For example in a given custom docbook instance I have a
paragraph or list where I would like to include a phrase or list item 
from a
given SPL instance.

Do both document types have to include identical XREF model?

Is it generally possible to set XREFs between instances that rely on 
different
DTD/XSD content models?

Thanks in advance,
Paul