I have two 7-month-old chinchillas. I got them at the same time, when they were about 3 months old. Their names are Hemingway and Fitzgerald.
Hemingway came from a breeder. He is well socialized and friendly.
Fitzgerald came from PetSmart. (I know, I know. I would not have planned to buy a chin from PetSmart, but it was one of those "wormed his way into my heart" situations.) He was small, sweet, and incredibly docile. Unfortunately, he hasn't stayed that way. As he got older, his personality drastically changed. He went from calm and docile to hyper and afraid.
With my wife and me, he's okay. I would not describe him as "friendly," but he is tolerant of our affection. He's willing to be petted, but he nibbles constantly (and will try to hump us any chance he gets).
The real problem is with Hemingway. When they get anywhere near each other, Fitz tries to claw and bite him. I wasn't concerned by this at first, assuming they just needed time to acclimate. However, their cages have been side-by-side for 3 months, and the problem hasn't gotten better. When their cages are just 2 inches apart, they're fine... but if that crucial 2-inch gap closes, the claws come out. I don't understand it.
Anyway, I have three questions for you fine people:
{1} Is it likely that Fitzgerald's aggressiveness will fade with age, perhaps after he finishes going through his hump-everything puberty stage?
{2} Is there anything else I can try WITHOUT giving them an opportunity to hurt each other?
{3} Ruling out abuse and trauma, because I'm not aware of either one occurring... what causes a really docile chinchilla to become paranoid and aggressive?