Introducing 2nd kitten not working

[ArielNeil]

Hey everyone, I write this to you in the midst of a feline crisis...
About a month ago we adopted a female kitty that was about 3 weeks old at the time, and named her Fleur.
Fleur is a very vibrant Kitty, thats always full of energy and the need to play, or else we become her play-prey, nothing too unusual for a kitty from what I’ve read (correct me if I’m wrong though).
yesterday we brought home a new male kitten, about the same age as Fleur, and we named him Albus.
Albus is an extremely relaxed kitten, I have never seen such a cat, he fully trusts his holder and always seeks touch, and he is very quiet and delicate.
For the first couple of hours we put one in a room and let the other roam the house, and switch from time to time.
Towards the evening we decided to open the door and let them both share the house, and that didn’t go well at all.
Fleur started hissing, spiting and growling and she backed away, and always kept a look to see what Albus is doing.
After some time she started chasing him and hitting him with her paws and she seems very aggressive.
At first he was only retaliating but now he also started hissing back at her, and the whole deal seems to be going down the drain....
Whenever they’re not separated they immediately jump at each other and fight until we separate them.
I’m dying for advices ,and maybe a reassurance that this can be turned the other way, and that they can be friends, because we really don’t want to take Albus back to where he came from...
Waiting eagerly for your advices, thanks!

[fjm]

Was Fleur three weeks or three months when she came to you? Three weeks would be extremely young, barely weaned.

i think you are going too fast on the introductions! Some very young cats get on immediately, but others need considerable time to accept an intruder into their territory. i would look at https://www.catchat.org/index.php/new-cat-introductions, and take things much more slowly, planning on it taking weeks rather than days.