Well, The reason (according to the same question answered in aviation forum) is that 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.
I didn't know the answer when opened the thread, when there was no quick answer I've further investigated. Thanks all for the replies.