Re: My two cats are going to kill each other!
Welcome, and I hope we can help you.
First, cats are *not* like dogs who can go away and return believeing everybody is their friend. Cats are much more territorial, though they can learn to live/exist together and tolerate each other. Removing a cat and returning a cat is not as simple as 'oh, it will be the same as it was before'. That almost *never* happens, unless the group of cats is *used* to cats coming/going for periods of time. Your cats have forgotten about each other and are each treating the other like an intruder.
You will need to start completely from scratch to repair this damage done. Please do some searches for "cat introductions" and/or 'reintroductions'. Go slowly, and you may find the pheromone diffuser "Fel-i-way" to be helpful, as it releases a smell that chemically signals 'calming thoughts' to the cats.
Best of luck,