The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:viewport match="element">
<p:with-input>
<result>
<element/>
</result>
</p:with-input>
<p:output port="result" sequence="true">
<p:inline>
<doc1/>
</p:inline>
<p:pipe step="producer"/>
</p:output>
<p:identity name="producer">
<p:with-input>
<doc2/>
</p:with-input>
</p:identity>
</p:viewport>
</p:declare-step>
Schematron validation
<s:schema 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/*[1]/name()='doc1'">The first child element of result is not named doc1.</s:assert>
<s:assert test="result/*[2]/name()='doc2'">The second child element of result is not named doc2.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
