I'm pretty I saw in the Isle of Armour trailer a multi battle at the Wyndon Stadium.
It was Raihan and Leon on the same team.
Actually, you did see correctly. After completing the Crowned Tundra, once it's released, you can compete in the Galarian Star Tournament. In this, you battle with and against in game trainers. This is the only time though that Multi Battles are available in Wyndon. Also, Multi Battles are available with Hop at various points in the game.