Hi Joanne,
It does sound like the cockatiel is getting most of the attention he needs from the parakeet. When birds bond, they often decide they are no longer interested in people. Usually this only happens when the share a cage, but in your situation, it is almost the same as sharing a cage since they are always out together. And since the cockatiel was not bonded with you in the first place, he found it easier to bond with another bird. When you let a bird have free roam, they tend to become less and less interested in human companionship. And again, especially if the bird was not tamed first. He is independent and only needs you to open the cage. When taming a bird, they need to rely on you to take them out of the cage and handle them. He simply has no incentive to bond with you because he has a bird companion and all of the freedom he wants. The only way to try to work with him would be to take these things away from him. I’m not sure that would be best for either bird. You can continue to leave everything as it is, and you will have to be content with any interaction he allows. If you separate the birds and stop letting him come and go at will all day, there is no way to know his reaction until you try it. He is likely to just pace and call for the other bird when he is locked in his cage. You may want to read our pages on behavior and taming and you might get some ideas.
Pet Bird and Parrot Behavior
Teaching Your Bird
Thank you for asking Lafeber,
Brenda