2 birds could mean 2 different personalities to take care of. I've introduced Jojo with Captain a few months ago and Captain stopped talking. Now she only talks when I put Jojo away from her in the other room where she can't see her. Their cages are side by side but they waste no chance to bite each other, even though, I think Jojo wants friendship more than Captain.
There are a few positives as well such as your bird will have constant company, they can communicate with each other, and if they are of opposite sex, then I guess their biological needs be fulfilled and they become happier and more content. This does not mean that a single bird may be less happy.
Negatives can be that your birds stop talking to you and will be more interested in each other, they may be adverse enemies to each other, the 1st bird may feel neglected, the 2nd bird may try to become the boss etc.