Only one mag (left) has an impulse coupler, which retards the spark for starting. If you activate both mags during starting, the non-coupled mag will fire ahead of the coupled mag, and you have a good chance of a kickback which can trash the gear teeth on the starter and the ring gear. Planes with a unified start/mag switch (key) have internal circuitry which grounds out the non-coupled mag when you turn the key to START. Planes with separate mag and start switches should be started with only the coupled mag activated; the other mag is activated after the engine starts. However, there are some aircraft with separate starter switches and Mag switches that do have circuits that ground the right mag while the starter switch is engaged. When in doubt starting on the left Mag only will help your starter live longer.