If you switch off the left mag and get a drop, ditto the right, neither mag can be permanently 'live' or you would get no drop when you switch it off. Ergo, no need to switch off both together then on again.
It's that last bit that can do harm. Some aeroplanes (such as the Chipmunk) don't have a lean cut off, so the engine has to be stopped by switching off both mags (and simultaneously going to full throttle). The latter obviates any tendency to 'run on', as incoming vapourised fuel is ignited by glowing carbon in the cylinder.