Oh goodness, I know your pain. I have 3 cats, 2 females that "tolerate" each other and act more like sisters, chasing, swatting and growling at each other and then my sweetie boy Sully who wants to play with the girls but they are utterly scared of him and when they swat he swats back and ends up hurting them.
Now I have gone to the lengths of getting an animal behaviourist to help with these problems. This was over 2 months ago and the progress is so slow that you really need a lot of patience! I was supposed to keep mine separated for 2 months by a gate that they could see each other through, which I sorta didn’t follow to the tea! I felt so bad for Sully being gated in the kitchen that I keep the gate open when I am there to supervise. I think our progress would be better if I hadn't done that but he is used to it now that he cries if I don't let him out once in awhile.
Back to your situation, what I have learned is if there is any and I mean any negative reactions in either of your kitties then you are not ready to let them be alone. Since I have been opening the gate we have had a few fights when I wasn't there (I trusted my husband to watch them, unfortunately not a good idea
) and every fight really steps you back a notch. Fights lead to bad association of each other and that is not what you want. Ideally you want to separate them as soon as you see something brewing, and I know that is not easy. Or, try distracting them by playing, that works for me. I bring out Da Bird and all the hissing stops!
I foresee a very long time, maybe years before mine will relax but that is what I am willing to do because all mine are rescue cats that I would never have the heart to rehome. I am sure you situation will get better before mine, my cats are much older then yours.
Cheer up, it will get better if you don't rush things!