I believe that the natural setup would be to have a 7" touch screen for entering commands and use a HDMI screen for output.
Actually I have already tried this with omxplayer as you can re-direct the video to the 2nd screen.
There is a few applications where I see this as a very good thing.
The QLC+ already has the capability to play video on the HDMI interface while the virtual console is run on the 7" official display.
But it would be a very good thing to be able to swap the screens. When you are planning your show you could use a full screen HDMI display to fine tune the lights and sound. And the video playback would go to the 7" screen.
The only problem with QLC+ is that I am in the middle of our performing season so my PiZero + AudioInjector + DiscoCap + 5" touch screen is happily providing lights and sounds almost every evening while I am on stage. So not much time to play with the code before the performing season is over. I already have a new Pi3B+ and plans for a new DiscoHAT. This dual display thing would be a great addition to that setup.
I also love the Fruitbox software with the old Jukebox skins. It would be cool to have a second screen for album art and video. Perhaps a second 7" screen just below the official 7" screen.
The 3rd project I am involved with is the RPi-InfoScreen project. As it is based on Kivy it should already support more than one screen. I just have not had the time to test it yet.
But having up to 4 screens is definitely a very good thing. Several small conning displays for gyro, log, wind, thrust would be great with a low-power Pi.
Here is a small example of what I have in mind... OpenGL rendered analog displays...