Using UE4.8.0 i discoverd a weird bug/feature.
While in Extended mode, dk1 legacy disabled , riftsaver enabled.
Starting your app and using Alt+Enter you will get your app on the monitor frozen and the app on the rift working.
When starting up your app using ~ and type Stereo On, will have the same results
When starting up your app using ~and type Stereo On [enter], Stereo Off [enter] Stereo On [enter] It shows the app on the rift and on your monitor.
So far i can tell the Stereo On command for the second time has the good working cycle, hope this helps some one