Can Civi/WordPress an Civi/Drupal (same versions) share a database?
Most of the database table can be shared between the two sites. However, there will/might be an issue with the WP user table and the drupal user table with the civicrm_uf_match table. One potential option might be to ensure that the user ids across wordpress and drupal dont conflict, i.e. choose a high starting point for the WP user table (which is a manual process in mysql)