When birds are first introduced to one another, there may be some initial sparring behavior, such as the wings-up posture and hissing in cockatiels, but, over time, many birds learn to accept a new companion.
Small birds, including cockatiels, have opinions about one another. To successfully introduce a new small bird to your household, start outside the cage, and don’t expect to deposit a new bird into an existing bird’s cage. All birds should have separate cages until their behavior proves otherwise.
Introduce your new birds in a neutral space like the back of a couch. See what they do and how they interact with each other. If they act aggressively (wings up and hissing, if cockatiels), you won’t be able to house them together.
If your birds eventually seem to get along in neutral territory and you cage them together, make it a roomy cage. Provide two of each food and water dish, so neither can monopolize a food bowl and starve the other. Watch them closely. If you see any sign of aggressive bird behavior, separate them.
With all of our pet birds, it is wisest to consider any new bird added to the household your bird. Birds don’t necessarily get along with each other. When you get a new pet bird, you get a bird that seeks your attention and affection, not necessarily a companion for your other pet bird(s).
I want to remind readers to quarantine new birds that come into your home. To avoid exposing your other birds to disease, keep a new member of your flock in a separate part of the house and “in quarantine” for a month to 45 days. Wash hands after handling the new bird, clean its cage last and during quarantine schedule a checkup by an avian veterinarian.