<opus-link>

<opus version="4.1"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="http://www.musicxml.org/xsd/opus.xsd">
   <title>Fidelio</title>
   <opus-link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="Aufzug1.musicxml"/>
   <opus-link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="Aufzug2.musicxml"/>
</opus>