Dc2 has cystitis, probably caused by stress. She and dc1 took a few months to settle down with each other, and were rubbing along OK until we went away for a few days. When we returned, they were immediately arguing a lot.
I've got a plug in thingummy. They don't share a litter tray, they both go out as and when they choose.
Would you seoerate them (which would mean dc2 not being able to go out when I'm out, and being shut wither in large front roomor upstairs. Dc2 uses a tray. Dc1 does not so needs access to the flap.