I'm using a Windows 7 Ion with 2 touchscreens.
I switched which monitor was the primary monitor. Then I changed the positioning from one on top of the other to side by side. When I exited Settings, both monitors were blank except for the mouse pointer. I had to power down by holding the power button.
I tried this twice with the same result.
If I recall correctly, I solved the problem by first changing the primary monitor, then saving the display settings, then changing the positioning.