We're beginning an effort to migrate from D6 to D8. Since the application being migrated is large, complex, and mission-critical; our plan is to port over and deploy small bits of functionality to D8 over time. Both D6 and D8 will be running simultaneously on production until a total migration of all functionality is complete.
Say what? Essentially, we'll be routing certain URLs to the D8 instance (ie: /featureB/), and leave the others to route to the D6 instance (/featureA/).
/featureB/ ----> Drupal 8
/featureA/ ----> Drupal 6