- Coordinate bilingualism: When two languages are learned in different contexts and maintained as separate systems
- Compound bilingualism: When two languages are learned in the same context and blended together cognitively
- Simultaneous bilingualism: When two languages are acquired from birth to age 3, with mostly equal emphasis given to both languages
And there are others: successive, balanced, additive, subtractive
This has to be extremely rare in today's connected world. For example our kid always went to british colleges while wife and I are native french speakers... but there's always been lots of english at home. We watch all our news and educational vids in english. We had our kid watch TV in english since the beginning (on purpose). So the "english as school / french at home" separation wasn't clear. (technically our kid did learn three languages and she was fluent in spanish for she lived for five years in Spain but then she "lost" it, even though she still understands spanish fine).
I mean: there's this thing that exists now and that is called the Internet and it happens to be quite ubiquitous, even in the third world. So I don't think a kid could ever be only exposed to one language at home, to a second language somewhere else (at school?) and yet parents would have missed the Internet revolution so much that they wouldn't even "compound" both languages at home? Parents stuck in the 18th century if you ask me. Even at a very young age there's already some "homework": it needs to be done, at home, in the language used at school. Youtube vids. AI voice assistants now. These things are simply a reality.
As TFA says anyway: it's even disputed if coordinate and compound distinction even exists at all.
I'm unsure how Internet and today's connected world is related to coordinate bilingualism. As an example of how coordinate bilingualism happens, my gf will speak to her mother in Kirundi most of the time. However, they both have the same job and when the discussion is work-related, they switch to French because of all the technical language that is in French, even though Kirundi is their native language.
Another example, my mother-in-law, again, will mostly speak in Kirundi with her siblings. However, when she or one of her siblings is recounting a story or a memory, they will switch to Swahili even if it's a second, or maybe third, language. She says that Swahili is "better" for storytelling and it's done naturally without thinking about it.
If you want a third example, for me there is a clear separation between French and English. To me, English is the language for internet forum and get access to more entertainments. It is definitely not a language that was ever used at home or work or that I was exposed to at home, not one time in the last four decades. And no, I don't live in the 18th century nor I missed Internet revolution. But again my brain isn't so colonized I'd send my kids to an English speaking school.