First, are they outdoor cats or outdoor indoor cats? I am wondering how an
outdoor cat would mess-up the carpet if it is indeed outdoors. Secondly, do
you have a littler box in the house? If so, then maybe if only one cat is
using your carpet as a potty, then she may need to see a vet. Just to check
her out medically. Thirdly, She may feel the house is just like outside and
needs to be marked as hers. Some cats that have spent their lives outside
find it hard to start using a litter box. They will if given time in a small
quiet area and a litter box. Finally, I hope this helped and I hope you find
the solution to your kitties problem.

Shadow Walker

Cat's usually eliminate inappropriately as the result of  behavioural or
medical issues. Medical issues should be ruled out first, so I
suggest you make an appointment with the vet for the cat in question and go
from there.

Judy

Has furniture been moved or replaced where she sprays? Have you changed
something in the house?

Are you leaving the cats in the same room together, alone? Try
separating them when you are not there and see if things change.