As I mentioned in my comment above, the ability to make the lock screen display the desktop background across both screens was removed in Windows 7. The closest thing I can suggest for getting around this is to use a photo screensaver capable of displaying on both screens (the default Windows one doesn't do that). You could even set it to use only one image for the "slideshow" if that's the only one you want it to display. I have used DisplayFusion for this. The screensaver option is not available in the free version (it's $25 for Pro) and really it's overkill just to solve this problem, but the program is nice and has some great features for getting the most out of your dualscreen setup. You can even set it to display your current background as the lock screen (though it still only shows on the primary monitor).