Multiple <tgroup/>'s break FO output

Hi,

I recently created a DocBook table similar to the
following one:

<table><title/>
<tgroup>
<colspec/
<thead/>
<tbody/>
</tgroup>
<tgroup>
<colspec/>
<thead/>
<tbody/>
</tgroup>
</table>

This is, of course, completely valid DocBook XML.
When I processed it to FO, it creates a fo:table that
looks like that:

<fo:table>
<fo:table-column/>
<fo:table-header/>
<fo:table-body/>
<fo:table-column/>
<fo:table-header/>
<fo:table-body/>
</fo:table>

which is not valid XSL:FO anymore.

Maybe this is a known issue, but I didn't find anything
about it in the docs or in older requests --- finding a
way to circumvent this problem would be great.

[I tried to think of a solution but did not yet come to
a conclusion.]

Best regards,
Johann Richard