introducing a 5 month old kitten and a 10 week old kitten - not sure whether to separate more or let them work it out
Hello!
We have a 5 month old resident female kitten and recently brought home a new 10 week old female kitten. The younger kitten was confident from the start and explored normally. Unfortunately, our RK escaped on the first day and went straight to our new kitten. She started hissing and growling constantly whenever she saw or smelled her.
After that fiasco, we started carefully separating them into different rooms and doing scent swapping by rotating rooms. Full separation is difficult because whichever kitten is alone in the smaller room eventually cries, and we do not live in a big enough apartment to comfortably separate both me and my partner with them so that neither kitten would be alone, so on Friday (we brought the younger one home Wednesday evening), we started supervised interactions during the day.
On Friday, they started using each other’s litter boxes without any problems, and they also started sharing a water fountain. After eating within one meter of each other without problems, we made the mistake of letting them stay in one room together (although supervised) for several hours.
We sprayed some catnip around the whole room to help them. At first it was fine. They mostly ignored each other. RK would sometimes watch NK, but she could easily be distracted into doing something else. They would stay about 1–1.5 meters apart without hissing, occasionally even closer. When NK suddenly got too close, RK would growl and hiss, but it did not escalate until the evening.
It all got worse in the evening. After they had eaten near each other, RK started chasing the younger one unprovoked in short bursts with a little hissing. That made the younger kitten hiss back. RK would then add a swing of the paw with claws, and the younger one would copy her again. We tried distracting them immediately, but it did not help. Even if we redirected them away from each other, they voluntarily walked back toward each other again until they got too close and the tension started again. We separated them for the night.
In the morning, we tried sharing a meal again. The meal went fine, but afterward it looped back to the Friday evening behavior. We separated them again for several hours with one room swap.
For the rest of the day, we did short shared sessions with feeding first and then playtime. Feeding continues to go fine within half a meter apart. During playtime they can sometimes get very close to each other (even NK passing almost directly around RK) without reacting, maybe only RK staring at NK a little sometimes, but she can easily be distracted with treats.
However, once playtime ends and we separate them (both in different rooms, doors open but only using a safety net - leftover from our window - to separate them), RK sometimes immediately, sometimes after a while, starts approaching NK more and more until they end up face to face, staring, hissing, and swatting. RK usually starts it, but NK has recently started hissing and swatting back too. Once that happens, we close the door to separate them fully, and RK stays around the door sniffing and watching, but not reacting in any other way.
My question ultimately is this: are we rushing them? Should we slow down more? After the Friday night fiasco, we always separate them after playtime before they have a chance to react negatively without toys and treats directly distracting them. Should we let them be together a little longer and work it out? Should we remove the phase with the safety net entirely and always fully separate them? At what point should we stop separating them when we are home?
We are new to having two cats at once, and we are afraid of pushing too much, not giving enough time, or misunderstanding what is normal boundary-setting versus actual aggression.
Thank you so much for all your help!