Wordpress and Drupal SSO/Single Sign On
You can use saml as your solution for single sign on and provisioning. Is standard and many companies uses it.
Check simplesamlphp , an open source software with good documentation, active project and nice comunity.
Exists plugins to easy add saml support to Mediawiki, Drupal and Wordpress.
Also google has saml support and simplesaml let easy handler it
OpenId would be a way to do it , other than that it should be possible to write a function to retrieve the Wordpress users info from the database and create corresponding users in Drupal. The function would then be called whenever a new user is created. Both platforms are well documented, so it shouldn't be too difficult to work out.
Now, I know you made your point clear about using both Wordpress & Drupal , but it does sound a bit like overkill knowing that whatever you can do in Wordpress should easily be possible in Drupal... but I digress ;)
You can check out modules that can allow you to do so. For example, I found this Drupal OAuth Login module and this OAuth Server plugin in WordPress that helped me with my use case.