<xsl:for-each select="document('first.xml')/source/object">
<xsl:variable name="object" select="."/>
<xsl:for-each select="document('second.xml')/order/attribute1">
<xsl:value-of select="$object/@*[name()=current()/@name]"/>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference