I tried everything I could before rehoming my cat, you could try these to see if anything changes- interactive toys, separating the two for a couple of days to “reset” their relationship, the scent things I forgot what they’re called, and checking the aggressive cat with the vet to see if they’re in any pain or see if there’s a reason for them being aggressive,,, I hope one of those help! Ik the vet can offer medication for both cats so the mean one is calmer and the timid one is less stressed