title: Switching between two UINavigationControllers
body: <p>Let's say I have two navigation stacks: one is for registration and login, the other is my main app.</p>
  
  <p>On first-launch I'm going to show the registration and login stack and on completion transition to the main app stack. How can I switch between these two stacks smoothly?</p>
  
  <p>Additionally, I'm going to show the main app stack and only present the login and register stack if the user signs out, so this one is clearer: just a <code>self.present</code> on the navigationcontroller.</p>