Seconding everything catalenamara said. Also, it can take up to six weeks until you notice the Feliway working. How many litter boxes do you have for them? Because that can add to the stress, as well.
But, for comparison, I have two eight year old cats, sisters from the same litter. They don't like each other very much. They had screaming fights every day, sometimes with tiny injuries. And it made them sick - every six months, one or the other suffered from cystitis. My vet gave me the following advice: Buy Feliway Friends, put up more litter boxes - one per cat and then one extra - and place them in different rooms/places all around the flat, and feed them special "Urinary Calm" food.
I've been doing this for about 1,5 years now, and it seems to help. They haven't been sick since (big yay!), and they're still not liking each other very much. They do growl at each other every day, but they can tolerate being in the same room together now, or even sitting about two metres apart from each other.
I'd say, give it time. It's early days, and they still can get used to each other or even become friends.
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But, for comparison, I have two eight year old cats, sisters from the same litter. They don't like each other very much. They had screaming fights every day, sometimes with tiny injuries. And it made them sick - every six months, one or the other suffered from cystitis. My vet gave me the following advice: Buy Feliway Friends, put up more litter boxes - one per cat and then one extra - and place them in different rooms/places all around the flat, and feed them special "Urinary Calm" food.
I've been doing this for about 1,5 years now, and it seems to help. They haven't been sick since (big yay!), and they're still not liking each other very much. They do growl at each other every day, but they can tolerate being in the same room together now, or even sitting about two metres apart from each other.
I'd say, give it time. It's early days, and they still can get used to each other or even become friends.