It sometimes helps to wipe all the cats in the house with the same cloth, say, twice a day, so that they gradually become used to each others' scents. A used T-shirt belonging to a household human is good; that gets the household scent on the new cats too.
I did this for a week when one of my cats got lost for several days and had difficulties reintegrating with the others when he came home (they treated him as a stranger), and it worked quite well.
I did this for a week when one of my cats got lost for several days and had difficulties reintegrating with the others when he came home (they treated him as a stranger), and it worked quite well.