Stubborn Habit or Just Mentally Challenged?

Through the years, we have shared our lives with quite a few cats and thier many different "personalities," but we have never had a problem with any of them persistantly urinating on our furniture until we got Haley, our female Calico.

Haley was about 7 months old when we took her in and did not give us any problems until we brought home Kiki, our other female who was just a four month old kitten at the time. It wasn't too long after that, Haley began urinating on our sofa and eventually our bed!  We tried everything we could think of. We took her to the vet to make sure that she did not have a urinary tract infection ~ she did not. We tried spraying our furniture with pet-safe products in an attempt to discourage her from eliminating on them ~ we were not successful. Our vet even suggested placing more litter boxes around the house as it might be a territory issue. But the problem with that idea is that we can not control which cat uses which box. The only way we could keep her box separate from the other cats was to put hers in a different room.

Fortunately for Haley, we have a nice little enclosed porch for her to call her own. We leave her in with us from the time we get home from work until the time we go to bed. However, she is still managing to urinate on our furniture when we are not looking. I am at a huge loss as to what more I can do about breaking her of this very bad habit?

Other than this frustrating little problem, she is a very sweet little kitty. She absolutely loves people and she doesn't have a mean bone in her body. We have had her for 6 years now and have grown quite attached to her so we feel getting rid of her is not an option for us. Any helpful comments or suggestions would be greatly appreciated!