Depending on the bird this is often the recommended practice.

There is always the risk that they won't like each other.

Or that they will like each other too much, and thereby no longer have any need for the human interaction.

Yup, that happened to my family. Probably for the better... Except one of them flew off the window when cleaning the cage. Then the other inexplicably flew off a couple days later. I imagine I should thank my parents for sparing us understanding depression at such a young age.

You are not legally allowed to get only one in for example Sweden.