Abilities » More Poison
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".
Source: Sword & Shield FAQ (2020-01-24), TPCi Rules Team (2020-02-06)
Abilities » More Poison | Attacks » Super Intense Poison
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.
Source: TPCi Rules Team (2017-08-03)
Abilities » More Poison
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.
Source: Burning Shadows FAQ (2017-07-20), TPCi Rules Team (2017-08-03)