How to share Drupal code across multisites that are each in subdirectories? I have two websites on my own server (I have full control of the Apache config, etc.)

I want to install Drupal 'into' each of them, in a subdirectory of the site root.

I'd like to share a Drupal codebase between the two.

And, I'd like to have the URLs appear without the subdirectory.

My problem is I can't figure out how to do all at once.

I can get to this easily,

/src/www
  /website1
    /drupal1
       ...
       sites/website1.com/themes/website1.com
       ...
    /other_stuff_1a
    /other_stuff_1b
  /website2
    /drupal2
       ...
       sites/website2.com/themes/website2.com
       ...
    /other_stuff_2a
    /other_stuff_2b


But I'm not sure how to share the Drupal7 code in this case.  Do I have to use symlinks or something?

I'd appreciate any help or ideas.

Frank
Drupal come with already a multi site feature, you ca read more about it here. 

After, if for some reasons you don't want install Drupal with multi-site and keep the both separate in your server, you can still use Git or SVN and co.

The idea is simple, you've the same codebase for development and multiple clone or checkout in your server. For example, in your case it will be drupal1 and drupal2.