It requires the following features: xslt-2.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="main" version="3.0">
<p:output port="result"/>
<!-- XSLT with secondary result documents -->
<p:xslt name="xslt">
<p:with-input port="source">
<document>
<primary>Primary</primary>
<secondary>Secondary 1</secondary>
<secondary>Secondary 2</secondary>
</document>
</p:with-input>
<p:with-input port="stylesheet">
<p:inline expand-text="false">
<xsl:stylesheet xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" exclude-result-prefixes="xs" version="2.0">
<xsl:output method="xml" encoding="utf-8" omit-xml-declaration="yes"/>
<xsl:template match="secondary">
<xsl:result-document href="http://example.com/{count(preceding-sibling::secondary)+1}">
<xsl:copy>
<xsl:apply-templates select="@*,node()"/>
</xsl:copy>
</xsl:result-document>
</xsl:template>
<xsl:template match="element()">
<xsl:copy>
<xsl:apply-templates select="@*,node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="attribute()|text()|comment()|processing-instruction()">
<xsl:copy/>
</xsl:template>
</xsl:stylesheet>
</p:inline>
</p:with-input>
</p:xslt>
<p:for-each name="add-base">
<p:with-input pipe="secondary"/>
<p:add-attribute attribute-name="uri" attribute-value="{base-uri(.)}"/>
</p:for-each>
<p:wrap-sequence name="primary" wrapper="primary-document">
<p:with-input pipe="@xslt"/>
</p:wrap-sequence>
<p:wrap-sequence name="secondary" wrapper="secondary-documents">
<p:with-input pipe="@add-base"/>
</p:wrap-sequence>
<p:wrap-sequence wrapper="documents">
<p:with-input>
<p:pipe step="primary"/>
<p:pipe step="secondary"/>
</p:with-input>
</p:wrap-sequence>
</p:declare-step>
Schematron validation
<s:schema xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:pattern>
<s:rule context="/">
<s:assert test="documents">The output root is not “documents”.</s:assert>
</s:rule>
<s:rule context="documents">
<s:assert test="primary-document">There’s no “primary-document” element.</s:assert>
<s:assert test="secondary-documents">There’s no “secondary-documents” element.</s:assert>
</s:rule>
<s:rule context="primary-document">
<s:assert test="document">There’s no “document” element.</s:assert>
<s:assert test="document/primary">There’s no “document/primary” element.</s:assert>
<s:assert test="document/primary[.='Primary']">The “primary” element has the wrong content.</s:assert>
</s:rule>
<s:rule context="secondary-documents">
<s:assert test="secondary">There’s no “secondary” element.</s:assert>
<s:assert test="count(secondary)=2">There’s aren’t the right number of “secondary” elements.</s:assert>
<s:assert test="(secondary[1] = 'Secondary 1' and secondary[2] = 'Secondary 2') or (secondary[1] = 'Secondary 2' and secondary[2] = 'Secondary 1')">The “secondary”
elements have the wrong content.</s:assert>
</s:rule>
<s:rule context="secondary">
<s:assert test="(. = 'Secondary 1' and @uri='http://example.com/1') or (. = 'Secondary 2' and @uri='http://example.com/2')">The URIs are wrong.</s:assert>
</s:rule>
</s:pattern>
</s:schema>