Cats need what seems like a LONG time for us to acclimate and decide how the tribe is going to work. Sometimes cats, at best, co-exist, but never really get along. However, I've also seen cats that seemed to hate each other become best friends two years later. I'd find them sleeping beside each other and grooming each other. Put the bully cat in another room to give the younger cat a break. Also, realize that they will work through it, but it literally can take months to a year. Cats can take their sweet time.