
Ideopathic cystitis, meaning there is no known cause. My favourite cat Vanilla suffers from it. In his case we can pretty much always relate it to stress. If something happens, he gets bad. We did for instance have one cat he hated and when I re-homed that cat to my daughter, he got a lot better.
One vital thing is to not feed dried food alone -any cat prone to cystitis or blockages from stones or crystals should be fed either on canned only, or one meal of canned and one meal of dried a day as it greatly reduces the risk of it recurring. It's all down to getting enough of a liquid intake. In fact force feeding water by syringe can help.
I'm wondering if these two cats maybe are fighting with each other so both are stressed?
I seem to remember my vet gave Vanilla steroid injections when he was at his worst, to calm it down.
It is very important always to keep a close eye on things and always consult the vet as if it should worsen to become a blockage, male cats can die from this within hours.
Hope this was of any help.