I have same problem,my solution is start the game on a single screen,after successfully opening the game, switch back to dual screen.
The game must be single screen when starting the game in VALK mode (if your "prop" file setting change to "G" or "H",not "F")
If you use two or three screens at the same time, it will not turn on .
I do not know why either
I have seen that other people can show sub monitor. But I don't know how