I don't think you can do that... At least not without the video driver having some kind of override function for the default Windows behavior.
You should be able to manually resize a window to cover more than one screen, but if you hit the maximize button, it's going to pick a screen and resize itself to that screen.
I have dual monitor connection. One is my laptop and the other one is connected to the dock. What I want to do is to be able to maximize 1 window to both monitors as if they were 1 big monitor.