I've introduced 3 pairs of cats in the past with success using some of the methods below, predominately room and scent swapping. All ended up co-existing happily in about a 4-6 weeks.
However, I've been trying to integrate 2 cats for the last 4 months. One has the lounge and the other the bedroom. Unfortunately, this pair are proving far more difficult. Both are non-socialised house cats. One was terrorised by a Chinchilla as a kitten. I've had one for 5 months and the other for 4. Both are spayed females. One is a 6KG British Short Hair Marbled Tabby, 5 years old and the other has some Abyssinian in her and is a Mackerel Tabby, 4 years old and 3KG.
Here's what I've tried with these 2, mostly in order, with a few notes of how it went in this case:
- Feeding both with a closed door between them:
Didn't work too well in this instance. One gulps her food down (she smells the other and is nervous) and promptly runs into the bedroom and honks it up.
- Socks for scent swapping:
If I leave the scented sock by the food bowl for one of the cats (trying for a positive association) she refuses to go near it. One has stopped growling at the others scent. The other goes ballistic.
- Plate swapping:
Small amount of food so they tend to lick the plate. Oddly, neither seemed to notice the others saliva.
- Room swapping:
One goes into full escape mode, including trying to tunnel under the door. Lasted over 2 hours yesterday. I was going in every 15 minutes to keep her company. Suddenly, a switch flicked and she calmed down. Started back up when I left the room. An hour later she calmed down and was laying on the bed. The other got high up in the cat tree in the other room and attacked when I tried to get her down. When eventually I got her back into her room she was her usual sweet self. When they switch into defensive/attack mode, only removal of the perceived threat gets them back into normal operational mode.
- Litter tray/box swapping:
Haven't tried this as one has a Litter Robot, but it would think it should help to acclimatise them to each others scent.
- Baby gate:
Installed after a couple of months so they could see each other. If allowed to get close, they will fight through it. It helps to minimise potential injuries, stops charging and prevents those 'I only turned my back for 10 seconds' incidences. One doesn't jump (overweight when I got her and on a diet) so at least she's contained and I close the room door when I'm not in.
- One cat in a carrier:
Tried this and took one into the others territory. She went into attack overload, so I got her back to base asap. Felt terrible for putting her through that afterwards.
- Feliway Deffuser:
Doesn't seem to have made any difference after a week of usage. Have the new Feliway Friends on order. Everything crossed.
New Feliway Friends / Feliway
Hope this helps