Linguistic anthropologists categorize bilingualism into a number of categories, and 'true bilinguals' are extremely rare.
I remember being in college studying linguistic anthropology and having a roommate who was unable to speak, read, or write beyond an elementary school level in FOUR languages, but could talk fluently in any of them.
Korean
Japanese
English
Italian
The guy's father was in the US Navy, so the guy was born in Korea and then moved all over the world throughout his childhood. Obviously his parents didn't seem to think it was so bad for him, but they were wrong.
By the time he started college, he was basically hopeless in terms of reading comprehension, writing, and even conversation skills. I think he flunked out.