Unless told otherwise, keep the cats completely separated from one another.
Cats naturally developed their preferred "territories" within a house. When cats are having conflicts, we ensure that each territory is appropriately stocked with necessary resources:
Provide water, food stations, resting locations/cat trees, and litter boxes in each cat's territory.
Create a common scent profile:
Use a soft blanket and rub the first cat with it. Focus primarily along the cheeks and base of the tail where scent glands are concentrated.
Repeat with the second cat as described above, then finish with the first cat again.
Repeat the same process but use the second cat as the first scent on the blanket.
Twice daily, feed high value food on either side of a french door or baby gate.
The distance should be such that both cats are able to eat, but are aware the other is present.
Over time, gradually decrease the distance until they are right next to each other, but remain separated by a physical barrier.
Our goal is for the barrier to eventually be removed.
Help the cats remotely play with each other under the door using specific toys with multiple attachments.