It doesn't work for me. I setup my second screen. Started the game and it only played on the main screen and left the other screen with the desktop background.
It will only use both if you make it think there is only one
"Horizontal span" makes a virtual screen comprising of both real screens side by side so the game thinks there is one big screen. If you dont then Windows will only let the game see the primary display regardless of how many you have.
What graphics card(s) do you have? If you use Nvidia check outh the Nvidia Control Panel (right click on the desktop, it should be listed) and select "Display", then "Change dosplay configuration" an select "As one large horizontal desktop (Horizontal span)", make sure you set the displays to correct order, Apply your changes and Bobs your uncle and SS08 should recognise the bigger display.
The only problem I have with it that in the middle of the display are the edges of the two screens......I wonder if you can have three displays......
Regards
Mark