Here's my logic on this; I'm sure others will differ.
If Fiji has not been exposed to other birds since you bought him/her and if you have not exposed yourself to other birds without washing yourself and your clothing before touching Fiji and if you have not purchased bird toys that could have been exposed to other birds who were shedding disease, then Fiji probably is disease free -- IF s/he was disease free when you purchased her/him. Regarding the second bird, if the breeder has her babies tested before selling them, neither bird would come to you carrying a disease. Most breeders who want to protect their own aviary will test all babies before they are sold because they need to have proof the bird was disease free when it left them.
If neither bird was tested for disease then it is up to you to decided if you want to get either or both of them tested. My opinion is that disease testing is much more important than quarantine unless you quarantine for 6 months to a year. Even then, some species are more likely to carry diseases which may not be shed until the bird is stressed, so even a long, long quarantine period cannot guarantee a bird is disease free.
There is another variable I forgot to mention. If the breeder acquired any birds between the time Fiji was sold and now, there is a chance the aviary was infected, IF the new bird was not tested and quarantined.
If you trust the breeder because the breeder keeps a closed aviary, then I do not think you need to have the new bird tested or quarantined.
