Many years ago, Puddles got a plastic bag handle stuck around her neck. She went tearing through the house with it, scaring Snowball to bits. Snowball did not recognize Puddles as his best friend, but something scary. For days after, Snowball would hiss and growl every time he saw Puddles (sans the bag of course
). Snowball would sneak around the house like there was a monster and Puddles didn't understand what was wrong. After a couple of days, Snowball settled down and they were back to normal. I did not isolate them as I knew it would take time and if Snowball went without seeing Puddles, he wouldn't get used to her.
I think cats live for the moment and can't reason like humans and events stick in their heads and what they saw at the moment of the event takes time to get over. I am sure in time, your kitties will be best friends. I also think that going on as if nothing happened because if you are tense when Zeus sees Ukko, he will pick up on those feelings and will be tense too.
Sending some