yeah, a Britain/Canada analogy had occurred to mewhat i'd envisioned when i typed "Principality of Brazil" was that, perhaps by tradition, the heir-apparent to the Portuguese throne stays in Brazil while the monarch lives in Portugal: when the monarch passes away, the Brazilian prince would then move to Portugal
The demographic pressure dictates however that should there be a junior partner in this union eventually it will be Portugal.
Portugal was more populated than Brazil in the early 19th century but it looks like the empire must still be ran from Rio to avoid a Brazilian rebellion. Both João and Pedro had that hindsight.