I modified tracker so that you add n files (and some
other classes) with one submit. But it doesn't work if
you try create more than one. Class instances are
created ok, but linking to 'owning' class fails. Only
one id is added to the multilink property.

I included a patch that seems to fix things, but I
might overlooked some conditions.

Problem with the original code (roundup/cgi/actions.py)
is that it each property can set only once (line 429).
So only the first id is added to the multilink property.

I realize this has been almost 10 years, but
do you happen to have the code that you implemented
that showed this problem?

To test and verify this isn't an issue anymore, I ran demo.py and
modified the tracker to use:

<tr tal:condition="context/is_edit_ok">
 <th i18n:translate="">Change Note</th>
 <td colspan=3>
  <textarea tal:content="request/form/msg-1/value | default"
            name="msg-1@content" wrap="hard" rows="5" cols="80"></textarea>
    <input type="hidden" name="@link@messages" value="msg-1">
    <input type="hidden" name="msg-1@author" value="admin">
    <input type="hidden" name="msg-1@date" value=".">
 </td>
</tr>

<tr tal:condition="context/is_edit_ok">
 <th i18n:translate="">Change Note 2</th>
 <td colspan=3>
  <textarea tal:content="request/form/msg-2/value | default"
            name="msg-2@content" wrap="hard" rows="5" cols="80"></textarea>
  <input type="hidden" name="@link@messages" value="msg-2">
  <input type="hidden" name="msg-2@author" value="admin">
  <input type="hidden" name="msg-2@date" value=".">
 </td>
</tr>

to basically duplicate the @note field (replace the exiting Change Note
named @note with the above). This should create two messages if you put
content in both text boxes.

When I do this, I see both messages created and linked to the
messages multilink for the issue.

So I am marking this works for me in absence of any other info
that I can use to reproduce your issue.