It requires the following features: xslt-3.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:xslt version="3.0" name="xslt">
<p:with-input port="source">
<doc/>
</p:with-input>
<p:with-input port="stylesheet">
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="3.0">
<xsl:template match="/">
<doc/>
<xsl:result-document href="secondary-true" build-tree="yes">
<xsl:sequence select="1, 'a', true()"/>
</xsl:result-document>
<xsl:result-document href="secondary-false" build-tree="false">
<xsl:sequence select="2, 'b', false()"/>
</xsl:result-document>
</xsl:template>
</xsl:stylesheet>
</p:with-input>
</p:xslt>
<p:for-each>
<p:with-input pipe="secondary" select=".[ends-with(p:document-property(.,'base-uri'),'secondary-false')]"/>
<p:cast-content-type content-type="application/xml"/>
</p:for-each>
<p:wrap-sequence name="false" wrapper="secondary-false"/>
<p:wrap-sequence name="true" wrapper="secondary-true">
<p:with-input select=".[ends-with(p:document-property(.,'base-uri'),'secondary-true')]">
<p:pipe step="xslt" port="secondary"/>
</p:with-input>
</p:wrap-sequence>
<p:wrap-sequence wrapper="result">
<p:with-input>
<p:pipe step="true"/>
<p:pipe step="false"/>
</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:ns prefix="j" uri="http://www.w3.org/2005/xpath-functions"/>
<s:pattern>
<s:rule context="/">
<s:assert test="result">The document root is not 'result'.</s:assert>
<s:assert test="result/secondary-true/text()='1 a true'">Results child 'secondary-true' does not have a text child '1 a true'.</s:assert>
<s:assert test="result/secondary-false/j:number/text()='2'">There is no number-element with text child '2'.</s:assert>
<s:assert test="result/secondary-false/j:string/text()='b'">There is no string-element with text child 'b'.</s:assert>
<s:assert test="result/secondary-false/j:boolean/text()='false'">There is no boolean-element with text child 'false'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>