XML IDREFs

This is more of an XML question.  I am going to make
this simple and talk with specific example:

I have 2 XML files one.xml and two.xml:

one.xml
------------------------------------------------
<root>
  <myelement id="coke">content for coke</myelement>
</root>
------------------------------------------------


two.xml
------------------------------------------------
<root>
  <text>
    Coke vs Sprite ... ... ... ...
  </text>
  <intxref refid="coke" label="Coke"/>

  <intxref refid="sprite" label="Sprite"/>
  <myelement id="Sprite">content for
sprite</myelement>

</root>
------------------------------------------------

I am validating both xml against a dtd and in my dtd
the intxref element is defined like this:
------------------------------------------------
<!ELEMENT intxref ANY>
<!ATTLIST intxref
      refid IDREF #IMPLIED
      label CDATA #IMPLIED
      >
------------------------------------------------

When I validate the two.xml file against the DTD, it
errors out giving following message: "A reference to
undeclared ID 'coke' has been found."  Note that if I
comment out the coke reference and leave just the
Sprite one in, then it works fine as the Sprite
reference is local within the file.  How would I do a
cross-reference with different xml files?  Is there a
way?  Is it even possible?

Thanks
C

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