Rehoming might be the answer. Tbh I've experience of cats who are stressing each other out, and locking cats in bedrooms while the other cats has a run around and vice versa was the solution there, but only as one owner worked from home and could police them. (House share).
You sound at your wits end, and I'm sure you will do what is best. Maybe try a specialist cleaner for the chair.
Thanks for your responses! Yes we thought cattery next time. We took her to the vet about her cystitis, she passes clots too :-( she is already on wet food and can't persuade her to drink more water - tried a fountain. He said he wouldn't investigate as is a clear relationship between her being harassed and stress cystitis - just suggested feliway which hasn't helped. She likes hiding under our sofa bed, not sure how I can get her a safe place away from the other cat? They only have one tray which is in the corner of our kitchen-diner, opposite end to the kitchen. She uses it happily most of the time (tends to wee over the edge but that's a different story!) Will get another one, don't really have room for 3 as would have to be in the kitchen so can mop up floor. Will look at the zylkene, thanks! Have asked my parents if they would take either cat (all expenses paid!) as they are retired and she could have a lot more freedom.
It may be that as she has got stressed out, it's making her bladder irritated and it's her way of dealing with it. I know it's hard to keep both cats apart, but it's worth trying to ensure she has her own space, not only from the other cat but somewhere she feels safe and can relax away from her humans.
What treatment does she has for her cystitis - is it just help as and when she has a bout? My girl used to get cystitis a lot, actually passing blood clots, but she was swapped over to a wet diet and has a maintenance dose of something called cystease. It's a natural remedy and you can buy it from the vet or online. She's been on it for four years, has given her no side effects and kept her cystitis free. She normally has one a day mixed in food, but we up it to two a day when we're away.
You ideally need one litter tray per cat plus one extra.
They need to be really clean, but no disinfectant used because some are toxic to cats plus some really pong. I use hot water and an ecloth these days.
It sounds like she needs lots of hidey holes so the other cat can't easily find her. Zylkene is brilliant for stressed out cats but it contains milk proteins which can upset some cats stomachs but it is easily available without prescription.
In future I'd find them a nice cattery with a pen each away from each other, it's cheaper than replacing furniture of course but having cystitis isn't fun for her either.
Have just got back from a week away, had a lovely cat sitter in looking after 2 cats. Younger female cat gets occasional stress cystitis and does little spots of bloody wee. This time she has obviously been emptying her entire bladder repeatedly on our lovely Laura Ashley fixed cover leather armchair :-( I am at my wits end, think it is because our other cat terrorises (rescued 2 so they could have company!) her she gets so stressed. Have tried feliway - no difference. Can't close off chair as open plan. Have ordered some enzyme wee cleaner but worried might have to scrap the chair. With a toddler to look after and working 4 days not sure I can cope much longer! Please help! This is my first post :-)