As a complete cat newbie, I am appealing to the wisdom of UKC.
We adopted our first rescue kitten 2 months ago (female, estimated 2 months old at the time). Everything went incredibly smoothly. She left the carrier after 5 minutes, explored the house, ate, used the cat toilet, and then snuggled up to us, all the space of a few hours. Since then she has been a pleasure, more like a dog than a cat, to be honest. Constantly appears with a ball or other toy and wants to play, sleeps in her bed all night, comes to us when called, etc. Even the vet visits have been easy.
We got a second kitten now (male, 3 months old, grew up with brothers and sisters in a protected space). Maybe our first experience made us overconfident, but it’s much harder to gain this one's confidence. The whole first day was spent hiding. After two days, he seems fine with me close by and is eating well, but does not let me touch him yet. This is fine, we just leave him his space. Biggest achievement today was licking my fingers on his terms. I pretended to be asleep and he just came over. As soon as I opened my eyes he was gone, but still a big step. He also calls us when we leave the room.
The problem now is how to introduce the two kittens to each other. They kept calling each other through the closed door, so we made one attempt to let them meet with a physical barrier (carrier). The older female wanted to be friends it seems, but the new one just emitted these very deep guttural growls, so we separated them again. Now the female keeps calling through the door but doesn’t get any answers and the new kitten just seems disinterested.
Any tips for smoothing the transition? Particularly on how to avoid any mishaps like fights? I have read that early encounters going wrong are hard to correct later on. Not really sure how to proceed here. Any tips on this and in general cat adaptation?