Everything Catty1 said is just perfect -- you need to get these kitties away from one another so that they can get used to each other slowly. Even when you DO introduce them again, the times together at first need to be just brief and as low-stress and happy as you can make them.

And don't underestimate the amount of TIME this is going to take! We want so much for our pets to love each other as much as we love them -- but they'll do it on their own time.

It will take weeks (or maybe even months!) of gradually getting them used to each other -- and even though it seems like a huge pain in the neck to do that now, it's the only way to go. Otherwise, they'll get used to their interactions being stressful with hissing and hostility -- and that is NO good, I promise.

Good luck!

Hugs,
Diana