I have 12 year old tabby female (adopted from 1year of age) and 3-4 year old black male (adopted last year). Straight off, she rejected him, did not eat for like 1-2 days and he was scared most of the time. Eventually he became more comfortable but their relationship developed mostly towards passive-aggressive.
They each minded their own life and when they sometimes got too close they smelled each other and she started hissing him. They were comfortable enough to eat/drink from the same bowl, next to each other, one after the other and to sleep in the bed at around 0,5-1m distance.
As medical info, they're both sterilised. She lost 1 of her hind legs in a car accident back when I lived at a house where she could come and go as she pleased (10 years ago). She has some sort of chronic bronchitis that makes her cough for a few seconds 3-4 times a day (tried different treatments from vet but nothing seems to work...she's fine before and after). He was so well cared (also seemed sterilised) when I adopted him that I really thought for the first 2 weeks that someone has lost him, so after spreading the word around nothing came up. Eventually 3-4 weeks in he had a bloody stool so that was his first visit to the vet, that eventually surfaced other problems. Apparently he wasn't sterilised, instead he had undescended testes. He underwent surgery to remove 1 of his abdominal testicle, the 2nd one was never found (probably disappeared after 3-4 years of age).
6 months ago I moved to another country, in a tiny 2 piece home with a nice little garden. I couldn't get both at the same time with me, so I brang him first and her a month later. They got used to the new home, they love the outside garden, but I keep them outside only under supervision (there are lots of stray cats around). 2 months ago they got fleas, probably from the stray cats, I started them on a monthly treatment with Fipronil and the problem was solved.
I never used a litter box. I taught my girl cat to pee and poop near the shower drain (11 years ago). Yes it smells, but it's a sign that I have to clean it up afterwards it's cheaper and she doesn't spread the litter sand all over the place.I did the same with him but I noticed after a while that he isn't very clean after he pees. He urinates in such a way that when the urine flows towards the drain it also touches his paws and then goes around the house for a bit before cleaning himself.
Coming back to the problem: The male, when he become more comfortable started entering play mode which ranges from scratching the pole, playing with various tiny objects (loves textile rubber bands) to running very fast from 1 side of the house to the other. And he does this a lot, having a lot of energy. She on the other hand, a lot less and it's usually playing with tiny objects and scratching. Now, he started provoking her trying to chase her during his "running" episodes and tried to somehow "play tag" with her with his paw. She interpreted this as an aggression and hissed/growled running away. This went on and on and turned slowly into mini fights that recently, in the last month she started also spraying him (with urine).
2 weeks ago I brought inside a cloth that I hang outside to dry and eventually fell from the wind. What I didn't notice is that one of the stray cats urinated on it. While I was trying to get some stuck leaves and herbs out of it, they approached me one at a time, being interested in the cloth. After smelling it a bit, they looked at each other weirdly and he made his specific "attack meow' and she her hissing/growl and they started a big fight. In the last week they started having frequent fights starting very similar to this, without them being in "play mode", just got close, stare at each other and fight.
I got scratches everywhere from trying to separate them each time, I also use a water spray that I started using a few months ago whenever they did a naughty thing. Thankfully, they don't have any physical injuries from all this fighting around.
2 months ago I had to leave them alone for a week, with someone passing every 2 days for food/water and litter box care. I had a camera installed to look after them and the mini-fights that they had at the time happened a few times when alone also. 1 month ago I started a new job that occupies me a lot more time, but my gf stays at home most of the time with them (she has to leave a few days a month though).
I tried my best to explain the background and the situation, if you have any other questions ask ahead. I don't know what else to do. Today they had 2 big fights after I got home and I'm currently keeping them separated.
Is it possible she might be jealous of him? He has all this energy and wants to play all the time and she is slow because of her lost leg? Idk, do you guys have any tips?
tldr; adopted 2nd cat 1 year ago, passive aggressive relationship at first, in the last months became more aggressive, in the last week they have big fights almost every day, what to do?
Later edit: I wrote this earlier this week and got some insight so far, but still don't have a solid long term plan on what should I do. I think there are 2 problems so far: 1. I never introduced them properly and 2. Girl cat feels a threat to her personal space by him wandering around with pee of his paws. So, I started with 3 litter boxes, that they seem to know how to use just fine. I still have them separated and put their food on different sides of the door (scheduled meals), I swap them every couple of hours. I managed to keep them together in a room with us during night-time when they're most likely too sleepy to fight. Today I got them both out in the garden, they each minded their business for about 20 min pretty far away from each other, then out of the blue she attacked him so...another big fight started. This is not going well at all...and we're getting injuries after injuries some pretty serious. What's the next step?