Everyone knows that if you enable your flash while taking pictures on your M8, you will not be able to apply the Duo effect on those pictures.
So if your flash is off when taking pictures, use a gesture to activate the flash.
Venom rom can map the flash to the volume buttons so that when you in landscape with the screen off, press and hold the volume button
to turn on the camera and flash together!
In this way, the pictures you take with the 'flash' on can be edited using the Duo effect!