Hi
It's difficult to give advice without seeing the cats and learning more about them. However, this occurrence is not uncommon and may take a few days for them to settle back down. Keep life as normal as possible and feed the cats a nice piece of ham each when they are in the same room. Use an old towel that both your cats have slept on to re-introduce their smell back into the house you can rub it around the chairs and along the walls.. You can pop Kitty's towel beneath the food bowl that Chloe eats from and vice versa so they begin to associate each other with nice things again. Do not get cross with either of them and keep an eye on them. If they are fighting badly you will have to separate them and gradually reintroduce them again slowly using mixed scent from each others old towels.
Is this stray still around? It might be a god idea to get your local cats home to try and remove it and re-home it especially if it’s a Tom cat (un neutered male).
If after a few days your cats are still unsettled it might be a good idea to arrange for a cat behaviourist to come and have a chat with you. But hopefully once the familiar smell has been re-instated and peace and harmony life will get back to normal.
Hope this helps
Rosie Barclay