I've had the long hair for 6 years and recently adopted the short hair (2023). This is the closest they have been to each other since. So much for getting another cat for companionship. I do work with them to try and build a relationship, but cats will be cats.
I would suggest a second opinion, if feasible. Also, I bought a couple of the "calming diffusers" with pheromones, etc. It isn't a fix, but I figured it can't hurt. Also, for working with them... They do enjoy my company. When they both come to me looking for scratches, I grab a couple "calming treats" to give them, in hopes that they get a positive reaction when they are together. Maybe see if that helps. I hope they get better!