Hi GenXu,
You’ll need to use the command Stereo On via a console command executed via Blueprint to trigger the switch. Alt-Enter, I believe, was disabled, but I could be wrong on that one. I know that Stereo On has worked for me previously.
This is also a good reference for VR. https://docs.unrealengine.com/latest/INT/Platforms/VR/CheatSheet/
-Tim