Is there a reason to use two separate databases for CiviCRM and Drupal, or can I use the same for both? What would be affected if I used the same for both?
Should work fine. Best practice is definitely two databases.
IMHO it is easier to understand where data is stored and deal with update and rolling back changes when the databases are separate, but in some hosting environments you are forced to use a single database for both the Drupal and CiviCRM tables.
Using separate databases requires you to go to CiviCRM > Administer > System Settings > CMS Integration and copy the array generated to your Drupal settings.php file.
Unless absolutely necessary, I never use the same database for both. Both Drupal and CiviCRM can end up with databases of a significant size. I've found that this can sometimes cause lag and slowness with the site. Not to mention that if you need to restore a database, it can be difficult for the average person to do once it gets over a certain size. Often times you'll need to get assistance from tech support at your host or know how to use command line.
I've found typically if I am so locked down that I can only have a limited number of databases, the server also has insufficient resources for running Drupal and CiviCRM well.