Most of the time, breeding or bonded pairs are a more interested in each other than they are their human companions. That's not to say all breeding or bonded pairs are mean or you can't handle them depending on the individual birds and how you interact with them, but you do loose that bond of being the closest "flock member" to your bird. Keep in mind, closely bonded birds interpret you as their mate, so when they find an actual mate or even another bird/pet/person/toy they interpret as a mate, you fall by the wayside at best, or they become outright vicious at the worst.
And MikeyTN brings up a good point. Some birds are like oil and water, and theres no guarantee they will mate or even like each other. My dad has had his male goffin for 39 years, and when he was a much younger bird, my parents apparently tried bringing home a female for him (they also have a closely bonded pair of amazons who are both friendly, and thought Alfie would also enjoy a companion of his own kind). Not so, he LOVES my dad and hates anyone who isn't my dad who gets even a millisecond of attention from him. He was intrigued by the female until my dad gave her some attention. Alfie immediately tried to kill her and they ended up having to return her to the petstore less than a week after they brought her home because even in separate cages, he would've gotten a hold of her at some point and done her in. Then there are bonded pairs like their male and female zons. Besides the occasional nip, they've gotten along for 40 years, you couldn't ever separate them they are that bonded yet are both still relatively friendly birds who enjoy human companionship as much as each others. Now, I don't know enough about toos to know if Alfie is just a special case, or if that's typical behavior, but IF you're looking for a second bird (of any kind) always have a backup plan in case they don't get along.
