multiple imageobject in imageobjectco or mediaobjectco

I think, imageobjectco should work in the same way as its non callout
equivalent when it comes to switch between FO and HTML output, but
apparently,  imageobject may not appear more than once in imageobjectco
nor does mediaobjectco with 2 imageobjectco make sense, because I would
have to define areaspec twice (hence running into trouble because of the
ids in "area")...

I would expect the following to work (or something similar):

    <figure id="fig-betra.general.main.window">
      <title>Elemente des Hauptfensters</title>

      <mediaobjectco>
        <imageobjectco>
          <areaspec units="calspair">
            <area coords="280 25" id="co-betra.menu" />
            ...
          </areaspec>

          <imageobject role="FO">
            <imagedata align="center" fileref="images/main-window.svg"
width="16cm" />
          </imageobject>

          <imageobject role="html">
            <imagedata fileref="images/main-window.png" />
          </imageobject>

        </imageobjectco>
      </mediaobjectco>
    </figure>


Thanks for any advice,
Georges