If I have two Toxicroak in play with the "More Poison" Ability, will my opponent put 4 more damage counters on their Poisoned Pokemon?
Yes, because the Ability says to "put 2 more" and not "instead of".



x2More Poison
Put 2 more damage counters on your opponent’s Poisoned Pokémon during Pokémon Checkup.


Poison ClawsFlip a coin. If heads, your opponent’s Active Pokémon is now Poisoned.
It bounces toward opponents and gouges them with poisonous claws. No more than a scratch is needed to knock out its adversaries.
If I have two Toxicroak in play with the "More Poison" Ability, will my opponent put 4 more damage counters on their Poisoned Pokemon?
Yes, because the Ability says to "put 2 more" and not "instead of".
Does Seviper's "More Poison" Ability stack with the effect of Toxapex GX's "Super Intense Poison" attack?
Yes, you first put 10 damage counters for the Super Intense Poison attack and then you get to add 1 damage counter for each More Poison ability.
Does Seviper's "More Poison" Ability stack if I have more than one of them in play?
Yes, each of your Pokemon with a "More Poison" ability will add an additional damage counter to your opponent's Poisoned Pokemon in between turns.