Hello and welcome to pet lovers! Don't worry, all it takes is time. Eventually kitten 1 will forget that kitten 2 was never not there
How long has it been since you brought the new baby in?
You can help things along by getting feliway plug in diffusers, and exchanging smells, by rubbing something that smells like you on one kitten, then the other. Put this shirt or whatever under kitten #1's food dish so the kitten will associate all the smells with something good.
Make sure you have some vertical spaces for the kittens to get up on.
In a room with a vertical space, sit on the floor with both kittens in the room. Engage in play with whichever kitten comes around first. The other kitten may start to play too, forgetting for moments at a time that he hates kitten #2.
The feliway plug in diffuser will help a lot. Plus....if they are not neutered, getting that done will help, too.
What are their names?