Breeding birds does not work that way. The cockatiels would need to get along to begin with, and then they would have to bond as mates. This means they need to be in a breeding cage, in a quiet, private place with no human contact other than when you feed them. You can no longer handle the birds or have them as a tame pet. If the birds do bond, then you give them a nest box and maybe they will use it and settle down and breed. If they bond and breed, they are now life partners and can’t be separated again. It would be cruel to separate a bonded pair. They would no longer have interest in humans or in being pets. They would only be interested in each other as companions. They are not like a dog, for example, that you can breed with another and go their separate ways. If you want to breed these birds, one of you has to give up their bird permanently, and both of you have to give your birds up as pets forever. It is not easy to breed birds by any means, and having a pair is no guarantee they will breed successfully. They almost always fail the first try. It can take months or longer for two birds to bond and want to breed, and it can often take a couple of years before they are even successful. Many pairs are never successful and many birds never make good breeders.
I would recommend buying a baby from a breeder if you want another pet. As for related birds breeding, in captivity, they absolutely will. They have no concept of being related, so you have to keep related birds apart if they are opposite sex.
Thank you for asking Lafeber,