buying a 2nd monitor is not really a fix ... but i managed to at least record some gameplay by moving the mirror window to 2nd desktop (winkey+tab) and using obs to just record that window ...
still this is way too complicated
I tried the oculus mirror thing, and it didnt work for me - while i get a window that would be suitable for watching / recording - my game doesnt take any input anymore if the windows isnt focused
edit: steam dirt 2.0 version in oculus rift mode