You are absolutely right, I meant Kivy.
What was tested was not that one app were using both screens at the same time, but that you could have desktop on one screen and the Kivy app was showing on the other. This was done by setting an environment variable called VC_DISPLAY. ( I saw it first mentioned here :https://www.raspberrypi.org/forums/view ... p?t=126156
So setting VC_DISPLAY=6 should get Kivy to show on official 7" display regardless of what is chosen for desktop. ( For any other type of touch display this will proobably not work. )
And I think then using the value 4 or 5 redirects to the HDMI..
So ONE Kivy app can not use both screens at the same time, but TWO Kivy apps started with different values of VC_DISPLAY should show on different screens.