Drupal Answers is the Q&A site about Drupal, but which of the questions about Drupal are on-topic on Stack Overflow, and then be asked on both the places?
As Stack Overflow is a programming Q&A, I would expect that any question related to how to use a module, which module to choose, which Drupal version to choose, should not be on-topic on Stack Overflow.
Once Drupal Answers is not a beta anymore, would Drupal questions be still accepted on Stack Overflow?