How do I implement two-way node references?
There are many modules available, your milage may vary:

BackReference (Drupal 6 only)

NodeReferrer (Drupal 7 beta release)

Node Relationships (Drupal 6 only)

Corresponding node references (Drupal 7 stable release)

Corresponding entity references (Drupal 7 dev release)

Content Dependency (Drupal 7 stable release)