I'm planning on moving some forums from vbulletin to Drupal. The problem is that I want to avoid adding more content to the current drupal database (MySQL) and I was wondering if its possible to keep the forums in a different database.

I know it's possible to have two drupal instalations sharing the same codebase (which is basically what I want) but how can I limit this second drupal instalation to be only forums (i.e. avoid users in the forum sections to navigate to any other non forum URL in the forums drupal installation).

Is it possible?

Thanks!