After following quarantine procedures, it is important to introduce the new budgie slowly. Try putting them in cages side by side for a week. If they are tame, you can then introduce them outside of the cages a couple of times. You can then try putting them together in the same cage and watch carefully for territorial behavior. You can expect them to establish a pecking order among themselves. It will help to reduce any territorial tensions if you have an extra food and water dish so they won’t have to squabble over an established eating/watering dish. Most likely both budgies will be very happy to have a new companion, but it’s still important to watch them for the first week. If you notice any extreme territorial action by one of the budgies, remove them to a neutral cage for a few days. Upon reintroduction to the same cage, he/she probably won’t be as territorial any more because he/she is now the newcomer.
How Do I Introduce a New Budgie to My Current One/Flock?