What if before the attack, when there is no hissing or growling,
The problem is that there is no point before the attack. The second they see each other the growling begins, it's instant.
How were these two integrated in the first place?
How long was it and what do you remember about it?
They shared the house for a few weeks before actually meeting each other. They were set up like they are now, during the day Tuna and Pocky get run of the house and then in the evening Corkscrew and Tibby get run of the house. They had shared the same cat trees, litterbox and water dish so we figured they would be ok.

We let them meet and as soon as Corsckrew saw Tuna they started growling. At that time Corkscrew was the aggressor, Tuna just wanted to sniff Corkscrew but Corkscrew went for an all out attack. That was when Alexa was bitten and had to go on antiboitics.

Every run in since then has been lots of fur flying and many times bloodshed. Tuna can sometimes be distracted by treats but Corkscrew will not give up. I've tried spaying him with a water bottle and he won't even flinch. He'll just stay there growling at Tuna allowing himself to get soaked.