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It is against ToS. But I know players in rural areas having 2acc to be able to play the game. If you have 1 gym and no other player your pokemon will not return with coins. So some players have another account with another color to get things moving at all. If this is all you do, you are probably not what Niantic targets.
Multiple Acc become a problem when they are used against other players e.g. to create tons of fake spots, keep other players out of gyms and raids...