I found it in a round about way too, could use some better publicity maybe. You can also use this to enable side-by-side 3D in VR, so like the old Nvidia 3D shutter glasses that made games pop out of a flat screen, you can see that in your virtual screen in VR. Take a look at SteamTinkerLaunch [0] to set that up (currently helping organize that giant readme, but the info is all there to play around with [1]).

[0] https://github.com/frostworx/steamtinkerlaunch

[1] https://github.com/frostworx/steamtinkerlaunch#Side-by-Side-...

Edit: I should say side-by-side mode works for games with it builtin (like Trine) but can also use shader injection to emulate this which can work pretty well too.