We were watching a video featuring two cats hissing at each other, at the end of which something funny was supposed to happen. We'll never know what it would be because, all of the sudden, our cats turned on each other with a violence we hadn't seen since we first brought home the youngest.
Imagine if you're in a room with someone else. You're not looking at each other. Suddenly, you hear someone say "You're an asshole".
Since you know that there's only one other person in the room, and you're not aware that sound recordings exist (cats aren't), you immediately assume that the other person called you an asshole. You'll likely respond by calling the other person a name.
The same is true of your cats. They likely heard the hissing, and assumed that the other cat (the only other creature capable of hissing in the house) was hissing at them.
This can quickly snowball. Cat A thinks that cat B hissed at it; so cat A gives an unfriendly reponse (likely hisses back). Now, cat B sees cat A hissing at it, so it knows that cat A is being unfriendly. And thus, a fight was started.
The problem is that it's a negative spiral. Thinking that the other cat is unfriendly towards you, causes you to act unfriendly towards it. The same happens for the other cat. In the end, they keep being unfriendly towards each other and perpetuate the issue, not understanding that neither of them started it (the video started it).
As to why your cats are currently fighting, there are two possibilities.
A. They are still responding to the misunderstanding.
This is the negative spiral I was talking about. This should die down eventually, but you'll need to negate the spiral (the negative feedback loop) before it does.
Annoyingly, you can't overdo it either. If you separate the cats (thus avoiding the negative feedback loop), they will learn to live without each other. If they are then reintroduced, it's likely that they never will have seen any redeeming qualities in each other.
In essence, while trying to avoid the conflict, you've put them is a situation where they only experience conflict.
The ideal solution would be to separate them, but make sure they see each other. For the sake of example, let's say you can build two glass cages in your house. I know it's not that easy, but I don't know your house or your resources to tell you what you can do. If you have e.g. a glass door, that's a possible solution.
This achieves the best of both worlds. The cats can't get to each other, so there's no point in trying to behave aggressively. They might do so in the beginning, but once they see the futility of their effort, they'll calm down and resume normality.
At the same time, the other cat sees them acting normal instead of aggressive. This plants the seed that the other cat isn't just an unprovoked aggressor. Seeing the other cat being calm.
You can drive the point home even more by making them contextual allies. Feed them at the same time, close to each other (with the glass inbetween). It may take a few tries, but the cats should eventually register that they're both trying to eat. They are proverbially breaking bread, which causes them to bond.
Another way to positively influence the bonding is to give them a common enemy. I achieved something similar with a laser pointer. Two of our cats both loved playing with the laser pointer, but were selfish about it. They also experienced daily social friction due to different characters/energy levels/playfulness.
The laser pointer solved many of those problems:
- It distracted the cats from each other, they were too focused on the laser pointer. This caused them to be near each other without focusing on their "disagreement".
- The older one would usually hate playing with the younger one because he was too energetic. However, she was experienced at chasing a laser pointer, which made her more playful than normal, thus bridging the gap between their differences in character.
- The younger one was much too energetic for the older one to handle (hence why she didn't play with him). But by letting him chase the laser, he tired himself out; thus making him more approachable. This again bridged the gap between their differences in character.
- After tiring both of them out, they naturally evolved a system of taking turns. One chases, the other rests. And if both wanted to play at the same time, they cooperated (trying to corner the laser pointer).
The older one was much better at tracking and swatting the laser pointer, and after some time, the younger one actually started learning her tricks. The two are now able to play together without needing my presence (the younger one controls his energy, the older one is more comfortable with playing around).
To summarize a few tips to ease the conflict:
- Make sure you're the center of attention, so the cats aren't thinking about each other.
- Let them break bread. Give them equal treats, food, affection, ...
- Try and swap their smells around (by petting both of them with the same hand, or swapping their toys/blankets once in a while.
- Try to find ways to get them to cooperate, e.g. a laser pointer. The core idea is that they need to see the other cat as an equal party that wants the same thing (just like with the food).
- If one of them is being unnecessarily aggressive, protect the other one. Make it clear that you're ignoring the aggressor.
- Don't be afraid to get physical. If one of them is actively causing problems, push them away from where they are. Make them feel like you're opposed to their current plan. Obviously, don't actually hurt or scare your cat, the intention is to give negative feedback about negative behavior.
- Try to lessen the chance of conflict. If you feed them together (without separation), make sure there is ample food so they don't feel like they have to fight ove the food. Make sure there are enough toys, and enough sleeping spots, so that they don't compete. Competition breeds rivalry.
B. The current conflict is based on actual fact.
For example: the initial conflict was based on a misunderstanding. The cats ended up in a (one-time) fight. However, during that fight, one of the cats may have ended up going overboard with the aggression. And now, the other cat sees them as dangerous, and refuses to let them come near.
That aggression is not a misunderstanding. It was clearly visible, and the other cat was the intended target.
This is much harder to clear up, because the realness of it means that the other cat is likely to remember it for longer. Depending on how much of a grudge your cat can hold, this may always be a scar on their personal relationship with each other.
The approach is the same as for option A. It's just going to take longer, and require more explicit proof that the cats are not each other's enemy.