Create a texture with texture flag 256 so it is stored in video ram. (This makes copying from video ram to it faster.)
Then each frame, render your scene in a small viewport, but do not flip, and copy the viewport to the texture you created.
Then set your viewport back to normal size, and place a quad over the viewport with the texture applied and scaled properly. Set the quad to add blend. Then render the scene and flip.
Then each frame, render your scene in a small viewport, but do not flip, and copy the viewport to the texture you created.
Then set your viewport back to normal size, and place a quad over the viewport with the texture applied and scaled properly. Set the quad to add blend. Then render the scene and flip.