Multiple Tracs with the Same User Database

For my master thesis I'm working with a couple of other people on quite a big project which consists of multiple sub-projects. To get some order into the whole process and documentation we are about to use Trac, the free project management tool by the folks at edgewall. It simply let's you handle one project per instance with its own SVN repository and user base. But what if you want to have multiple instances with the same users but just different permissions per instance?