Two extended magnets - screws included.
With an extra set of magnets, one for each wheel, you will get even more light out of the SL200, as the light will flash faster. At a normal speed of approximately 20 km/h, the flashing frequency is approximately 400 flashes/minute with two magnets and approximately 600 with three magnets per wheel.
The SL220 flashes at a fixed flashing frequency of 120 flashes/minute, and adding an extra magnet will not change that. However, with an extra set of magnets, the PowerBackUp function in the SL220 recharges much faster, which is clearly an advantage if you cycle in cities and often have to stop at red lights.
I also recommend that you mount an extra magnet in the rear wheel of bikes with Nexus – i.e., bikes that have seven or eight internal gears if you have lights from the SL200 series. This is due to the configuration of the spokes, which makes it difficult to mount the magnets optimally.