From what I know, the moves Soak and Magic Powder change the target’s type, while the moves Trick-or-Treat and Forest’s Curse add a type to the target.
Therefore, if you were to use Soak/Magic Powder and then Trick-or-Treat/Forest’s Curse on the same target, they should stack, turning the target into a Water-Ghost, Water-Grass, Psychic-Ghost, or Psychic-Grass type, depending on the moves used. This would be useful in a multitude of ways, including making a Normal type 4x weak to Ghost moves (which Gourgeist, the only Pokémon that can use Trick-or-Treat, would undoubtedly have many of).
The question is: Do the games allow you to stack these moves, or will the second move fail on a target affected by the first?
(I’d like an answer for all applicable games in gens 6-8, but if you can only answer for one, please answer for SwSh.)