A couple I knew wanted to live together, but they each had a cat. Now, when you bring a cat into another cat's territory there's often fireworks. However, they both left their apartments, and moved into a new apartment, on the same day. Cats just looked at each other. Got along right away. 
Very unusual and a risk
Advice from experts
be sensitive to what a cat might experience
take time
give space
terrific guidance below
icatcare.org/articles/int...
There is a theory about cats who are not super friendly with each other inside, will band together to defend against any outside critters from coming inside.