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I wanted to create a new pokemon (For example a Diglet which is ground-ground) that was four times weak against water type and two times stronger at using STAB moves.
I think that you'll be best off treating your Pokémon as if it had an ability that doubled type-effectiveness (e.g. look at how Solid Rock etc work) and gives it more powerful STAB (look at Adaptability). You don't actually have to create an ability for this—just check for the species in the places where you'd normally check for an ability.
Also, I don't think it's super relevant but if you actually managed to give it two types that worked in the obvious way (i.e. all computations involving type run twice), double-STAB would be 2.25× because it's 1.5× applied twice.
I think that you'll be best off treating your Pokémon as if it had an ability that doubled type-effectiveness (e.g. look at how Solid Rock etc work) and gives it more powerful STAB (look at Adaptability). You don't actually have to create an ability for this—just check for the species in the places where you'd normally check for an ability.
Also, I don't think it's super relevant but if you actually managed to give it two types that worked in the obvious way (i.e. all computations involving type run twice), double-STAB would be 2.25× because it's 1.5× applied twice.