I went through the animal introductions with 2 cats - our old solo female initially refused to even go in the house if the new rescue male kitten was inside - if she saw the kitten, growls, snarls, etc.
However, slow intro, extra-favorite treat food (tuna) on opposite sides of a gate so she'd associate something good with seeing the kitten. (Ever see a cat dig into tuna alternating with looking up and hissing?) Took several months, but eventually the female tolerated the kitten and even allowed him to groom her.
So it may take a really long time, but I know it's rarely an option to give away one or the other once you've become attached to both.