Schema constraints from a different file?

[Dmitry Epstein <mitia@northwestern.edu>]

I have a data-oriented project, in which the main data component
is an XML catalogue of items, supplemented by a number of other
XML files that specify additional information about some of the
items from the main catalogue, or reference particular items in
descriptions of other data.  Right now this is accomplished by
giving each item in the main catalogue a sequential number, then
referencing that number form other files.  I am trying to
introduce Schema validation into this project, and I am
wondering how I could validate item references in files other
than the main item catalogue.

For example, suppose the main catalogue contains elements that
look like this:

<item>
         <itemID>ABC123</itemID>
         ...
</item>
<item>
         <itemID>DEF456</itemID>
         ...
</item>
...

Then in another XML file there are elements that look like this:

<itemSpecs>
         <itemID>DEF456</itemID>
         ...
</itemSpecs>
...

Basically, I would like to develop schemas that impose
constraints on an element or attribute in one XML file based on
element or attribute values from another XML file.  Is this at
all possible?  I would consider making a file containing just a
list of all available item IDs, if that would help solve the
problem.

TIA