Cockatiels are a bit like potato chips, you can't have just one!
But the prospect of introducing a second cockatiel to your spoilt and people orientated baby can be a bit scary! Here's how i do it, hopefully it will be helpful if you're thinking of adding a second or even third, fourth, etc cockatiel to your flock!
Always remember to quarantine any new bird for a minimum of 30 days in a separate cage and a separate room to any existing birds. This is VITAL to prevent the spread of disease in your flock. No matter where you bought your bird from, please do not take the risk - ALWAYS quarantine!
After a 30 day quarantine period of juggling your time between two or more birds, i'm sure your eager to get them all together so they all get more time out. However, it's important to take things slowly.
I always start any birdy introduction with meeting through the cage bars. Once the quarantine period is over you can put the cockatiels cages side by side so they can get used to seeing each other. They will probably be curious about their new neighbour, and may sit on the side of their cage which is closest to the other cockatiel.
Next you can get them out one at a time, and let them climb over one another's cages with the other safely behind bars. As long as they're not trying to kill each other through the bars you can probably start getting them out for supervised play time together.
I find that the first play session together goes smoothest with a nice millet spray to break the ice. Everyone is too busy scoffing millet to be too bothered to bite the new friend, or pull their crest. A good first playtime leads the way to a good friendship!
After this point you need to move at your cockatiels' own pace. You will want to start letting them go in the same cage during the day, and once they're both entirely happy with their new friend you can start leaving them together unsupervised, and finally full time living together. For some cockatiels this may only take days, and others may need to make the change gradually over a few weeks. You know your tiels, and they know when they're comfortable! So watch them, and read their body language.
As long as you don't push the relationship, introductions should go smoothly and without too much drama!