Let's make sure we are talking apples and apples. The exhaust tips (covers) are clipped on to the tail pipes (slides over) on the 535 but the tail pipes are welded on to the muffler. So if you are going to replace the singles with doubles you will need to cut the tail pipes off and weld the dual tips on. If you were swapping the chrome (or black) tips (covers) then they can be twisted on and off.
2011 535i xDrive, Dark Graphite Metallic, Veneto Beige Dakota Leather, Technology Package, M Sport Package, Executive Package delivered Christmas Eve '10, Mods - quad tips, 20% tint, JB4.