Can you put in a new cockatiel with an old cockatiel?

Yes you could but they might not always get on. I have got a cockatiel. I did have another one but she died. :( Anyways , when I put my new cockatiel, Polo, in the cage the other cockatiel Biddy just hissed a bit and went to the other end of the cage. The sometimes used to fight but not dangerous fighting. Just bickering at each other. If they do start fighting and it could lead to... well, you know what I mean, you should move one out of the cage for a bit and maybe put them in another cage. Put the cages next to each other so they slowly get used to each other. That's what I did. Now Polo is lonely because Biddy died :( . anyways, hope that helped! xxx