Sara is right. Babies have fits because they cannot control their temperatures. I had them too when I was a baby and I'm 100% fine. So your babies are going to be OK. Just keep an eye on them when they get temperatures and give them Calpol or whatever is recommended now to lower their temperature.
Most of all, stop blaming yourself for being worried. Seeing anyone having a fit is disturbing to say the least, to see you precious baby having one is absolutely terrifying. Cut yourself a bit of slack and allow yourself to be a bit overprotective for a week or two more. I hope you have spoken to your health visitor/GP and that they have reassured you that there is nothing really wrong with your baby. If you haven't, get on to them. They should be sympathetic, especially as you have post-natal depression.
I think someone should have a word with your husband, as shouting at you when you are having a panic is not helpful. I'm sure you would give anything not to have panics. Would he prefer it if you ignored his children ? Or maybe he was really worried by the fit too. Men sometimes react with anger and denial if they are frightened by something. Just a thought.
When you wake up at night and feel the need to check on them, could you have a look at the clock and try to make yourself wait for five minutes before you do go and check ? If you can do that, after a few days of doing that extend the time to ten minutes and so on. Hopefully you might fall asleep while you're making yourself wait.....all this lack of sleep isn't helping you either.
You are not being over the top, you are not a nervous wreck, you are a Mum coping with two tiny babies who has had a dreadful fright.