They can be kept alone if given enough attention. That meams, a couple hours out of the cage daily. Not all of this time has to be spend actually interacting with the bird - you could for example, let it out to play on a gym or sit on your shoulder when you watch TV. If the cockatiel will be all alone during the day, you should try having a radio on quietly for some ambient noise. Of course, if you have more than one they will keep each other company. You would still have to give them plenty of attention if you want them to stay tame and friendly with you.

Karen
Chico (conure) and Pippin (budgie)
WI

I've had several tiels. They are actually better pets if they have a bonding period with you before getting them a companion. I interact with mine several times a day. First thing in the morning I give him the opportunity to spend time with me if he desires. He spends a lot of time with me in the evenings. Ussually most tiels will fast pace their cage when they want out.