When it comes to two male cats living under the same roof, issues and tension generally arise, perhaps from competition over the prized window perch or the attention and favor of a beloved owner. Thankfully, neutering tends to change cat behavior for the better.
Unfixed male cats are driven by their hormones, and as a result can get very territorial and competitive with others. Allowing two unneutered male cats to live together is often a recipe for disaster, especially when an unfixed female in heat is nearby! If you neuter your two boys, you likely eliminate all traces of problematic mating behavior, including territorial urine spraying, aggressive fighting, loud vocalization and wandering and restlessness.
In some rare cases, neutering doesn't entirely cut out problematic behavior among male cats. This is more likely among cats neutered at a later age. And if you introduce a new cat into your home, your neutered boy still may react badly—such as by spraying urine everywhere to show the newbie that he's boss. With urine marking, a cat is essentially saying to another, "This is my turf. I own this couch, and the human who sits on it. Back off, or else!"
If you adopt a new cat and want him to live in peace with your older resident cat, make sure to conduct your introduction correctly. Before you let your new cat see your other pet, allow him some time and space to adapt to his unfamiliar environment away from chaos and distraction. Give him his own special space that your established cat can't enter. After a few days, try to start acquainting your cats with each others' scents, first by sharing toys, blankets and other items. To enforce happy associations with each others' scents, reward your kitties with yummy treats. When you do finally introduce your new cat to your other cat, make sure that Junior is safely confined to a carrier, and be sure to keep your eye on the event closely! Growling and hissing at first is natural, though it should subside with time. After a few of these successful "confined" meetings, gradually start allowing your cats to meet and interact in a more normal setting. Make sure both cats are able to hide or flee if they feel threatened by the meeting.
Overall temperament is the main indicator of whether cats can get along, neutered or not. Quiet, calm and relaxed adult felines, for example, may have a much more successful time with similarly mellow pets. With a sprightly, mischievous and rambunctious younger cat, however, problems could arise. When it comes to cats liking or even tolerating one another, you must use your best judgment, based on what you know of all the participants.