How to get two dogs to get along?

We recently acquired a second Jack Russell Terrier. The first dog is a four yr old female, fixed, named P. The second dog, the one we recently acquired, is a six yr old female, fixed, named H. H is aggressive towards P, and has to be kept in a crate. P is submissive towards the older dog, even though it is P’s territory. We are giving it three days, and if things do not improve, H will have to go. We have already tried trying to get them to ‘kiss and make up’ by holding each one, but H is still damned aggressive. H came from a house with four other dogs; P has lived here almost three years, single dog home, but she gets on well with other PT dogs. She is a trained PT animal.

Does anyone have any suggestions on how to get the two dogs to get along, or is it just going to be one of those things of letting them get used to each other?