My husband and I have gone thru this a few times, in fact we are still doing so. What we did was put the new one in a room of her own for a few days. Then, and I know this isn't possible for everyone, we have the capabilities to divide our house but still let everyone view each other. Did that for a few days as well. This past Saturday all the doors opened and life has been...interesting to say the least. I know every time we have gone thru this in the past it has been scary. The sounds they can make we have had only one 'failure' and Sam really is much happier as an only cat (but we have visitation rights!!) All I can say is that I agree with the few people who have said give it time, and I also recommend the ear plugs and a squirt bottle or two.

Best of luck!