Provided the cats get along well I believe that indoor cats are better in pairs for the simple reason that they can entertain each other and keep each other company thereby providing mental stimulation. I believe that the biggest difficulty with keeping cats full time indoors is ensuring that they are entertained and stimulated. They live in an artificial environment. A more natural environment will automatically stimulate a cat. The touch, feel, sounds and scents of nature are all excellent ways to stimulate the cat’s mind. This is all missing in a full-time indoor cat although if the cat is allowed to go outside in a cat enclosure then he or she will be able to experience those outdoor sensations. Although hunting may be impossible and hunting is the prime activity of a cat. All play is about simulating hunting.
A further problem for an indoor cat is that the owner may well be out for a large part of the day and therefore the cat will be alone. I believe that this is not a good situation for a cat to be in. He or she may suffer from separation anxiety. He may become stressed and stress can result in health issues such as cystitis. Another cat with which he gets along very well will probably remove these potential issues. The key, though, is that the cats must get along and it is not a foregone conclusion that they will. Neither is it a foregone conclusion that siblings will get along when they become adults.
However, cats have the ability to form friendships with other cats as we know. I don’t know of any easy way to ensure that your cat gets along with another cat. Of course, even if a pair of cats don’t get along initially they will become more tolerant of each other but I’m not sure that is satisfactory. It is not a solution to the single indoor cat but a good cat friend is.
Do you have personal experience of indoor cats living together and getting along well? I have to own up and say that many years ago I had a female cat who I had to leave alone in my flat at the time because I was working full-time. She was a great cat. She became stressed although I wasn’t fully aware of it. She developed cystitis. I am more aware of these issues these days. My veterinarian at the time put me straight.
There should be some sort of method to test whether cats can get along before they are adopted. What I mean is that the resident cat should be able to meet the incoming cat for a period of time, under observation, to see whether they can get along rather than simply adopting another cat hoping for the best because if goes wrong I think it can be disastrous. There have been occasions when I have heard from visitors about cats not getting along and the only genuine solution has been to rehome one of the cats. This is not a situation one wants to find oneself in and the same goes for the cats.