or are they not in one game, or can't be used by one Pokemon? what if a Pokemon leech seeds another and then switches to partner eevee, who uses sappy seed?
Leech Seed and Sappy Seed both plant a seed on the target.
The move will fail to seed the target when used on a Pokémon that is already seeded, or that is currently Grass-type.
Source
So no, Leech Seed and Sappy Seed won’t stack.
Sappy Seed and Leech Seed are not able to be on the same Pokémon Partner Eevee is the only Pokémon that can learn it and it can’t learn Leech Seed. Leafeon can learn Leech Seed but Partner Eevee can’t evolve in Let’s Go and if it’s transferred to another game, Sappy Seed has to be forgotten and can’t be remembered. You can have another Pokémon with Leech Seed and then switch to Eevee but in the end, the two moves can’t be on the same Pokémon.