How to run multiple sites through a single install?

I want to create a question-answer site for students using OSQA for italian students. I installed OSQA and it works fine.
I have created several subdomains (like history.university.it, english.university.it, math.university.it, etc) and i added to al this websites the osqa web app. The problem is that when I open a question, for example in history.university.it, i see this question in all the other websites, the same question.

So I tried to following this guide but it is not clear and I failed, i get 500 Internal Server Error.
I tried to ask the question at osqa.com, but I have not received any replies. I use the Webfaction server so I decided to ask here.

Thank you for taking the time to look at my question, and I hope to receive a good response.
Thank you very much! ;)

Edit:

What I did:

• I created a copy of the files settings.py and settings_local.py for the new site in the same directory as the main osqa site and I've renamed them in settings_math.py and settings_local_math.py and I edited them accordingly.

• In my osqa.wsgi I specified sys.path for the new settings. Below the
  os.environ['DJANGO_SETTINGS_MODULE'] = 'osqa.settings'
  i added
  os.environ['DJANGO_SETTINGS_MODULE'] = 'osqa.settings_math'

• Now i edited the settings_ math.py and under the
  #User settings i added
  from settings_local_math import *

• I created database and filled settings_local_math.py accordingly and I added APP_URL (i.e. http://math.university.it/)

• I run python2.7 manage.py syncdb --settings=settings_math (I don't get any error here). Then I restart apache with ~/webapps/osqa_server/apache2/bin/restart

• Finally from the browser I enter the website address and I get 500 Internal Server Error.