Running two Drupal sites off the same database: Is it possible? Is it possible point a second Drupal site to an existing Drupal database, if that second site is only used to browse the content, no logins, registrations or content add/edit/deletes?
Is it possible? Yes. Should you do it? Probably not.

Using table prefixes, you can run two completely separate sites with table names like "site1_node" and "site2_node", so they don't collide. In your case, it sounds like maybe you also want to share data between the sites?

On solution to that, is to share tables between these two, but I strongly recommend against that. I've tried it on two different sites, and I've mostly gotten bit badly by it. Use Domain instead, to run two domains from a single Drupal install.