New kitten in the house

[Kiwikinon]

Hello !
I took in a lost kitten two days ago (we might keep him if we don’t find the owners) and I already have a kitten that is around the same age as him which is great, but the introduction process was really messy.
I live in an apartment with my family and I tried to keep them separated but they keep sneaking out, chasing each other and even using each other's litter boxes ! Sometimes they lay next to each other calmly but the resident cat usually end up running away ... all that in 2 days, it’s been really crazy lately !
I read the dedicated article on cat introductions, so I try to use the advices on positive association, meal and playtime and most of the time the resident cat like to observe the new comer which is normal I guess.

The new kitty is the lovely and very friendly, but the resident cat keep hissing and hitting him (without his claws so it’s fine but I stop him if he does)
I’m sure they are gonna be good friends in the future but I would appreciate any advice on how I can make it easier for the resident cat at this point ? (Knowing that I can’t keep them separate at all times, especially at night)

 

[Kurisunium]

Good luck to your new kitten, I hope you enjoy them.

 

[Mamanyt1953]

From what you are saying, this is perfectly normal behavior for kittens...even kittens who know each other! Play, for a kitten, is all about learning hunting skills, so there's going to be bouncing and pouncing and swatting and, yes, occasional howls of outrage. They are not trying to hurt each other, or the claws would be out. Let them have at it. Don't worry about those swipes, they are all part of the game. Think of two small boys wrestling around. It looks much rougher than it actually is. The fact that they sleep next to each other says it all.

 

[Kiwikinon]

I guess it went quite well then ! But unfortunately the owner took him back ! I’m so sad he left, because we were ready to keep him.
The owner was kinda mad we took him in because apparently he’s « used to staying outside », yet he followed a stranger to our residence, and cried all night in the cold, my neighbor discovered him around 6am but their cat was really agressive so we took him in. He has 2 siblings that are also lost, I hope that they will find their way back home. My poor baby I hope he will be fine (between us, if I ever see him again, I’ll take him home, It’s too dangerous to let a kitten roam around in the streets)