[modeless]

You absolutely can shoot parts of a film in 24 FPS and parts at 48 FPS. Instead of transforming the 48 FPS parts to 24 FPS, you do the other way around and transform the 24 FPS parts to 48 FPS, not by doubling the speed but by repeating each frame twice. In fact, film projectors have always displayed movies at 48 FPS with frame doubling to reduce the appearance of flicker: http://en.wikipedia.org/wiki/Frame_rate#Background