Mixing sources to create a pattern

I have two standalone xml files, strucure.xml and data.xml.  I also
have the XSLT file layout.xsl.

layout.xsl is being applied to structure.xml, and data.xml is being
used in layout.xsl like below:

<xsl:variable name="data" select="document('data.xml')/data" />

In layout.xsl i'm in a loop below:

<xsl:for-each select="some_tag">
    <div id="{@id}" class="{$data/@id}">...</div>
</xsl:for-each>

So if structure.xml has <some_tag id="test_id" />
and data.xml has <data><test_id>test_class</test_id></data>
I want a transformation of <div id="test_id" class="test_class">...</div>.

Why does this not work and is their a solution to create a pattern
mixing these different xml sources?

Thank you.

--
Andrew Ferk