It requires the following features: xslt-3.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" xmlns:xs="http://www.w3.org/2001/XMLSchema" version="3.0">
<p:output port="result"/>
<p:xslt version="3.0" template-name="start" name="xslt">
<p:with-input port="source">
<p:empty/>
</p:with-input>
<p:with-input port="stylesheet" expand-text="false">
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="3.0">
<xsl:output build-tree="no"/>
<xsl:template name="start">
<xsl:result-document href="dummy.xml">
<xsl:variable name="document">
<doc/>
</xsl:variable>
<xsl:sequence select="map{'one' : $document, 'two' : $document }"/>
</xsl:result-document>
<primary/>
</xsl:template>
</xsl:stylesheet>
</p:with-input>
</p:xslt>
<p:identity>
<p:with-input pipe="secondary"/>
</p:identity>
<p:identity xmlns:map="http://www.w3.org/2005/xpath-functions/map">
<p:with-input>
<result>
<map-check>{. instance of map(*)}</map-check>
<map-size-check>{map:size(.)}</map-size-check>
<map-of-nodes-check>{. instance of map(xs:string, node())}</map-of-nodes-check>
<map-of-document-check>{. instance of map(xs:string, document-node())}</map-of-document-check>
<map-item-is-document-check>{every $item in .?* satisfies $item instance of document-node()}</map-item-is-document-check>
</result>
</p:with-input>
</p:identity>
</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="result">The document root is not 'result'.</s:assert>
<s:assert test="result/map-check/text()='true'">The value of map-check is not "true".</s:assert>
<s:assert test="result/map-size-check/text()='2'">The value of map-size-check is not "2".</s:assert>
<s:assert test="result/map-of-nodes-check/text()='true'">The value of map-of-nodes-check is not "true".</s:assert>
<s:assert test="result/map-of-document-check/text()='true'">The value of map-of-document-check is not "true".</s:assert>
<s:assert test="result/map-item-is-document-check/text()='true'">The value of map-item-is-document-check is not "true".</s:assert>
</s:rule>
</s:pattern>
</s:schema>