I've been thinking about this too.
I have come to the conclusion that it can only be done backwards.
That is, by having the flash actuate the shutter.
I think so too. If you use the camera to trigger the flash, it will start when the first curtain has finished its travel. By this time, with a shutter speed faster than its flash sync. speed, the second curtain will already have started its travel.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.