Yes, this is quite normal. However, I would double check and be absolutely sure they are both males. Sometimes mistakes have been made, and you don't want unexpected chinchilla kits
But assuming they are both male, then you are seeing a normal dominance behavior. One Chinchilla will "mount" another, as if they were mating, as a show of dominance. It is also a frequent behavior seen during play. It sounds like you have a well-bonded, happy pair of boys.
