Instead of running the screen saver directly, DisplayFusion will create a window that's the size of your screen, then call the screen saver in preview mode and pass the handle of the window to it. That's the only way to make the screen savers span both monitors, or to run a different screen saver on the second monitor. Generally, most screen savers can only run on the primary monitor without using preview mode

Some of the newer built-in Windows ones (like Ribbons) actually work across multiple monitors without preview mode. There's such a wide range in the way that screen savers work, so the settings that work for some don't always work for others