Mmmm, seems like Ziggy is a little irritated. I'm wondering if he thinks Zuzu was the reason he got sick.
I found a couple of articles on our own site that explain cat movements and communication. Maybe by reading them and understanding what the movements mean it would be easier for you to interpret what is going on. To me it seems Ziggy wants to play but Zuzu is not reacting the way he expects. Therefore he gets upset.
My suggestion - keep one in the safe room during the day or when you are at work. When you are at home and can watch them let the one out of the safe room and watch the interaction. If you see Ziggy start to approach Zuzu in a threatening manner clap you hands loudly. What you are doing is trying to divert his attention from her. You could then get out an interactive toy and play with them both at the same time. Maybe a separation from each other for the better part of the day and at night is what they need.
If all else fails you could try a calming drug like Prozac or Valium. It may be enough to calm them both down and let them settle.
You mentioned you already have Feliway. It does take up to a month to kick in. Do you have it in every room they are in or just the one they share the most? I would be tempted to have them in all the kitty rooms.
Has Ziggy been back to the vet for a recheck? I'm wondering if he is still not feeling 100%.