cat week's on the stairs, help

[blamethecat]

We have two cats one who is 14 and had since a kitten and one who is 7 and we adopted at 18months, the younger one has always had a tendency to wee in the house when the weather is bad , but now she has picked a spot on the stairs and wees there at least once a week. She has access to outdoors via a cat flap, two litter trays with different litters in which get completely emptied one a week and solids removed asap, .I am getting sick of cleaning the carpet and am worried there will be serious damage (rented house) . I clean using carpet shampoo and have tried bicarb to neutralize the smell and washed with bio washing powder. Can't fit any more litters trays in due to toddler and actual space. She does prefer one litter over the other and it did help but not completely. Any ideas ?