Until February 19, 2023 at 3:59pm PST / 6:59pm EST (23:59 UTC), you’ll be able to fight against both Sylveon and Garchomp in 4- and 5-star Tera Raid Battles in Pokémon Scarlet & Violet. Unlike previous two-Pokémon raids, both versions are able to fight both Pokémon, as they are not version exclusive.
These two Pokémon are pretty iconic Pokémon of their respective types and, while they don’t offer particularly compelling rewards according to Serebii.net’s coverage page, you may still enjoy taking a swing at them and catching them, either for your own collection or for training up.