If one or both cats are hissing, take a step back in the introduction process. Put your cats in separate rooms with separate bowls, beds, and litter boxes. Keep their food on either side of the same door to create a positive association to the smell and sounds of each other while they are eating, but not interacting. Every day have the cats swap rooms, this will help them get comfortable with each other’s scent. Once they are comfortable on either side of the door, you can allow them to see each other at a distance. And, if they are comfortable, you can consider another face-to-face introduction. Never hesitate to take a step back in the process. Moving slowly is the best way to build a healthy relationship between your cats.