I'm with these guys. Before I came to medical school I had a ring necked parakeet, was out of the house a lot for work, so got a CAG to keep him company. The pair of them hated each other, so I couldn't have them out of the cage at the same time. I couldn't even let one out while the other was caged if I wasn't in the room - so in the end Cookie, the parakeet, got less attention than when he was alone,
I think if you've got more than one person in the house, it's fine, but if it's just you I'd stick to one bird, and just try to enrich it's environment as much as possible.
Julia