From the answer in this question, it looks like type effectiveness for dual-type Pokemons can be "stacked up together".
So if a Pokemon's attack move type is super effective towards both types of a dual-type Pokemon, then it is 1.25 x 1.25 = 1.56 effectiveness.
The cases of
1.56 vs 1, or 1.25 vs 0.8
are easier to think of, such as Vaporeon vs Arcanine, or Lapras vs Dragonite, but what about the cases of:
1.25 vs 0.64 1.56 vs 0.8, or even 1.56 vs 0.64
Do they exist in Pokemon Go?
P.S. I think I see a case with a Bug type Pokemon vs an Exeggutor having
1.56 vs 0.8. That's because a Bug move is super effective towards both Grass and Psychic type Pokemon, and a Grass move is not very effective towards a Bug Pokemon. So an Exeggutor using a Seed Bomb fighting a Scyther with Bug type moves will be
1.56 vs 0.8. Unfortunately, Bug type Pokemons tend to not have high HP and CP, and their charge moves are not that powerful either. I think the strongest is Bug Buzz which is 22.1 DPS (there is no Bug type charge move that is 30 or above).