You can buy another cockatiel - especially if they are "cage-bound". Do you take your bird out of it's cage at least 1 hour a day? If not... Please buy another cockatiel! If you do buy another, try to buy from a breeder. Look on the internet for "Aviculture Societies" in your area. They can refer you to a reputable breeder. Bring the new bird home and place it in a separate cage next to your current cockatiels cage. Let them get used to each other for a few days, This is so important! It may cost you a little more to buy a second cage, but if you want a happy couple - it's the right thing to do. Your bird will bond to the new bird after awhile. Don't let it hurt your feelings. It's God's way. If one of them lays eggs in their food dish... Contact me, that's an issue all it's own.
Do research on this. I always thought Cocktail were loyal to their owner. I know that they're great pets. Hopefully it'll work out with two.