如何使p:xslt聚合来自两个源的输入

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc"
                xmlns:c="http://www.w3.org/ns/xproc-step"
                version="1.0">
  <p:input port="source1" primary="true"/>
  <p:input port="source2" primary="true"/>
  <p:output port="result" primary="true"/>

  <p:document href="source1.xml" port="source1"/>
  <p:document href="source2.xml" port="source2"/>

  <p:xslt>
    <p:input port="stylesheet">
      <p:document href="transform.xsl"/>
    </p:input>
  </p:xslt>

  <p:wrap-sequence>
    <p:input port="wrapper">
      <p:inline>
        <wrapper/>
      </p:inline>
    </p:input>
  </p:wrap-sequence>
</p:declare-step>