Using an XSLT 2.0 processor like Saxon 9:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="xs"
version="2.0">
<xsl:param name="doc2-url" select="'test2014032603.xml'"/>
<xsl:variable name="doc2" select="doc($doc2-url)"/>
<xsl:key name="id" match="Document" use="Tags/Tag[@id = 'document_id']"/>
<xsl:template match="@* | node()">
<xsl:copy>
<xsl:apply-templates select="@* , node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="Document[key('id', Tags/Tag[@id = 'document_id'], $doc2)]">
<xsl:copy>
<xsl:copy-of select="@id, key('id', Tags/Tag[@id = 'document_id'], $doc2)/node()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
With XSLT 1.0 it is also possible but to switch contexts between documents for the key use the code ends up being convoluted:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">
<xsl:param name="doc2-url" select="'test2014032603.xml'"/>
<xsl:variable name="doc2" select="document($doc2-url)"/>
<xsl:key name="id" match="Document" use="Tags/Tag[@id = 'document_id']"/>
<xsl:template match="@* | node()" name="identity">
<xsl:copy>
<xsl:apply-templates select="@* | node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="Document[Tags/Tag[@id = 'document_id']]">
<xsl:variable name="this" select="."/>
<xsl:for-each select="$doc2">
<xsl:choose>
<xsl:when test="key('id', $this/Tags/Tag[@id = 'document_id'])">
<xsl:for-each select="key('id', $this/Tags/Tag[@id = 'document_id'])">
<xsl:copy>
<xsl:copy-of select="$this/@id"/>
<xsl:copy-of select="node()"/>
</xsl:copy>
</xsl:for-each>
</xsl:when>
<xsl:otherwise>
<xsl:copy-of select="$this"/>
</xsl:otherwise>
</xsl:choose>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>