Merging two XML files using XSLT

Given the following input files:

ApplicationData.xml

<?xml version="1.0" ?>
<ApplicationData>
    Whatever data you have in here.
</ApplicationData>

MetricList.xml

<?xml version="1.0" ?>
<MetricList>
    Whatever list you have in here.
</MetricList>

AssessmentInput.xml

<?xml version="1.0" ?>
<AssessmentInput />

the following transformation merge.xsl applied to AssessmentInput.xml

<?xml version="1.0" ?>
<xsl:transform
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    version="1.0">
    <xsl:template match="/AssessmentInput">
        <xsl:copy>
            <xsl:copy-of select="document('ApplicationData.xml')" />
            <xsl:copy-of select="document('MetricList.xml')" />
        </xsl:copy>
    </xsl:template>
</xsl:transform>

produces the correct output of

<?xml version="1.0" encoding="UTF-8"?>
<AssessmentInput>
    <ApplicationData>
        Whatever data you have in here.
    </ApplicationData>
    <MetricList>
        Whatever list you have in here.
    </MetricList>
</AssessmentInput>