Switched food and water bowls so the cats could get used to each other’s scent.
Had New Cat (NC) switch spaces with Resident Cat (RC); NC spent most of the time in the kids’ play area, while RC was confined to the master bedroom/bathroom.
Put RC in the dining area and closed the French-style doors leading to the play area; placed NC in the play area. RC hissed and growled while looking at NC through the glass; NC sat and stared at RC (and didn’t grow or hiss).
At night, we closed the master bedroom door and let NC roam the master bedroom and bathroom, while RC has the rest of the house.
It looked as if RC stopped eating.
Separated the two cats like we did on Sunday (using the French-style doors); RC growled and hissed when she saw NC through the glass.
NC got a new litter box with a dome and a swinging door; switched her to the cat-litter brand RC was using.
Separated the two cats like we did on Sunday and Monday; RC and NC startled each other, and then RC growled and hissed. Afterward, I held NC, wife held RC, and we stood six feet away from each other; neither cat hissed or made a sound.