Did you get one cat before the other one so the first one thinks it was his/her home when you brought the second one in? The same thing happened to me when my cat was 7 years old and the only cat and I got a kitten. At first, the older cat was fine with the kitten, kitten wanted to play more than the older one, but soon got the message if the older was not in the mood. When the kitten got to be about 2 years of age, all of a sudden she turned on the older one, making those low threatening growls, hissing, chasing her, and the older was was so docile she was very scared and let the 2 year old dominate her and hid in the basement. We finally brought the older one up to the living room, along with her food and her litter box and she stayed in that room only for 2 months, as the younger one wouldn't let her out. After 2 months, the older one started venturing out a room at a time and the younger one acted ok. I believe what happened is the younger one wanted to establish that she was now the alpha cat and as long as the older one knew that, all was fine. They even became friends again and the older one lived to a ripe old age of 17 and died of diabetes complications. So, hang in there, the same thing may be happening.