Thank you! I think I encountered this problem in the past, too.

I just pushed these changesets, the third one was needed due to code
duplication for links and multilinks.

changeset:   4846:3e36a3bc0335
user:        Thomas Arendsen Hein <thomas@intevation.de>
date:        Wed Nov 13 11:12:21 2013 +0100
files:       CHANGES.txt roundup/cgi/templating.py
description:
issue2550822: Fix showing more than one additional property in class menu

Report and patch provided by James Mack:
| I had the following option in one of my templates:
| tal:content="structure python:context.propname.menu(additional=
| ['propname1', 'propname2'])"
|
| I expected this to display both propname1 and propname2 in the menu. It
| only displayed propname2.


changeset:   4847:843886af0ea1
user:        Thomas Arendsen Hein <thomas@intevation.de>
date:        Wed Nov 13 11:12:55 2013 +0100
files:       roundup/cgi/templating.py
description:
Reformat previous change (3e36a3bc0335) to 80 chars


changeset:   4848:e8ae54db8b95
user:        Thomas Arendsen Hein <thomas@intevation.de>
date:        Wed Nov 13 11:13:26 2013 +0100
files:       roundup/cgi/templating.py
description:
Fix issue2550822 (3e36a3bc0335 and 843886af0ea1) for multilinks, too