this sounds like something i went through with my cats. Give them both a little time and if they still dont get along find one of them a good home. The bottom line is that some cats are not going to get along no matter what you do. It maybe heartbreaking to have to decide which one goes but it may be best for the health of the kitten that hides all the time. kittens that live in constant fear of another tend to be less healthy. With mine we already had three and one more just showed up on our doorstep. She wanted to stay so bad that she clawed at the windows till her paws went bloody. We took her in and she fit right in with two of them. The other one just did not want her there and she attacked her everychance she got and we were afraid that our cat would kill the new one so she had to go seeing as we already had our three cats fixed. it was hard but it had to be done.