The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="main" version="3.0">
<p:input port="source">
<doc1/>
</p:input>
<p:input port="secondary">
<doc2/>
</p:input>
<p:output port="result"/>
<p:try>
<p:wrap-sequence wrapper="result">
<p:with-input pipe="secondary@main source@main"/>
</p:wrap-sequence>
<p:catch>
<p:wrap-sequence wrapper="result">
<p:with-input pipe="secondary@main source@main"/>
</p:wrap-sequence>
</p:catch>
</p:try>
</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="local-name(result/*[1])='doc2'">First child of result is not named 'doc2'.</s:assert>
<s:assert test="local-name(result/*[2])='doc1'">Second child of result is not named 'doc1'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
