Before making this decision be aware there is the chance the two birds will never get along or that they will get along so well they will not want you around. Sometimes two birds will get along wonderfully and still have a nice relationship with their human but that is not always the case. I haven't usually heard of a cockatiel killing another one like I have heard in lovebirds or some other species but they will sometimes peck, bite, or starve the rival by not allowing access to food or water.
First the new bird would need to be quarantined for 30-60 days even if thought to be healthy.
Then the two need to be introduced slowly. Bring their two separate cages into the same room so they can see and talk to each other. After a few days if all is going well, situate the cages next to each other to allow possible close interaction, or grooming through the bars.
IF this goes well introduce them outside the cages in neutral territory (maybe on the back of the couch a new playgym etc.) If that goes well for a couple supervised meetings then you can try placing them into a neutral cage together.
If you are planning on using one of the same cages then it will need to be made to seem a different cage. First scrub down the cage, place new toys inside. Move the food and water bowls to different positions, move the perches up or down or even back to front if possible. Then for temporary place it in a different area of the room. This way neither bird should see it as "their" territory and feel the need to protect it, it will be neutral turf for them both. Try to supervise closely for the first couple days like maybe on a weekend etc. If all goes well then great!!