"Two cats are ready to be introduced as long as neither is showing any signs of stress or exhibiting any aggressive behavior, like hissing, growling or swatting," Hovav said. "Signs of stress include poor appetite, inappropriate urination or defecation or excessive hiding."
As long as neither cat is exhibiting any of these behaviors, they can be introduced through a slightly cracked door. If that goes well, they may be ready to meet without a barrier between them.
Give cats space.
If one cat becomes aggressive, do not try to calm or soothe them, Hovav advised. "Give them some space and keep them separated from the other cat."