void Start () {
Debug.Log("displays connected: " + Display.displays.Length);
// Display.displays[0] is the primary, default display and is always ON.
// Check if additional displays are available and activate each.
if (Display.displays.Length > 1)
Display.displays[1].Activate();
if (Display.displays.Length > 2)
Display.displays[2].Activate();
}

这样就可以了..原来要还要激活一下呀..用Unity 运行的时候不行..一定要导出后在PC上才能看到这样的效果