Make interaction between multisites
The "standard" way of creating multiple sites with some interaction between them using Drupal is to use the Domain Access module rather than multisites.

However, there is great write-up about using interacting multisites at Drupal Redesign: Domain Access vs. Multisite.

The gist of it is that you set up a multi-site as you normally do. Then you create a custom module to take care of interaction and sharing data between the sites.

Drupal can handle multiple databases, but you can avoid the complexity of managing multiple databases by using a single database and use a database table prefix to avoid namespace collisions.  If you already have created the databases as separate databases, you need to use hook_update_N (or SQL operations outside of Drupal) to refactor the schemas.