Auraninja
Eh, ragazzo!
That's probably going to be a Home thing, but we don't know anything quite yet.I don't know if this was asked before. Can you trade Pokémon that are available in both between Sword and Shield & Brilliant Diamond and Shining Pearl? If so, how? By having two Switches to trade between or with something like Pokémon Home?
My guess is that they'll go in detail in the next (possible) Pokemon Direct of Feb. 27, 2022.