Thanks very much! It works fine. And it doesn't even matter, which one of the groups is defined as primary group, at least not for those users, who have both groups asssigned to them. Important is to have both groups defined as secondary.
Nevertheless, one issue remains. No matter, which of the two groups is assigned as primary, the upload space allowed is not shown in the users control panel (remains "0"). But this is mentioned in the help screen.
However, this is only a simple setup. Having a more complex system would require something like an access matrix, where you can assign each option per category, album and user. The settings panel could look like those assessments one gets after consulting hotlines