From my point of view you don't have to use two different design directories.
In setting/siteaccess/(my_sitaccess_xy)/site.ini.append.php just set the design directory to the same value.
[DesignSettings]
SiteDesign=my_design #(e.g. in case of wizard-generated shop site this is "shop_site")
AdditionalSiteDesignList[]=base
However, I don't know the impacts of changing these settings while a large site is already running...
I made the experience that a common var directory can have advantages as well in terms of cache handling (otherwise I had the effect that different languages were difficult to keep up-to-date, obviously because of remains in different caches.)
Can somebody confirm this please? Or am I totally wrong...?
You can set up only one design form your multilingual sites.
Look at the documentation to assure your language configuration is correct. http://ez.no/doc/ez_publish/technical_manual/3_8/features/multi_language
After that use your design settings for siteaccesses:
I'm not sure why, but I couldnt make this work. So before spending hours at this, I gave up and used a symlink instead - wich works for me. Thanks anyhow : )